HeaderDoc support in the library and generated sources

- Convert most of the core library headers over to HeaderDoc format.
- Switch the generated comments over to HeaderDoc.
- Create GPBCodedOutputStream_PackagePrivate and move some things into there
  that should be more internal.
diff --git a/objectivec/GPBUtilities.h b/objectivec/GPBUtilities.h
index 5b55104..b720932 100644
--- a/objectivec/GPBUtilities.h
+++ b/objectivec/GPBUtilities.h
@@ -38,24 +38,34 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-// Generates a string that should be a valid "Text Format" for the C++ version
-// of Protocol Buffers. lineIndent can be nil if no additional line indent is
-// needed. The comments provide the names according to the ObjC library, they
-// most likely won't exactly match the original .proto file.
+/// Generates a string that should be a valid "Text Format" for the C++ version
+/// of Protocol Buffers.
+///
+///  @param message    The message to generate from.
+///  @param lineIndent A string to use as the prefix for all lines generated. Can
+///                    be nil if no extra indent is needed.
+///
+/// @return A @c NSString with the Text Format of the message.
 NSString *GPBTextFormatForMessage(GPBMessage *message,
                                   NSString * __nullable lineIndent);
+
+/// Generates a string that should be a valid "Text Format" for the C++ version
+/// of Protocol Buffers.
+///
+///  @param unknownSet The unknown field set to generate from.
+///  @param lineIndent A string to use as the prefix for all lines generated. Can
+///                    be nil if no extra indent is needed.
+///
+/// @return A @c NSString with the Text Format of the unknown field set.
 NSString *GPBTextFormatForUnknownFieldSet(GPBUnknownFieldSet * __nullable unknownSet,
                                           NSString * __nullable lineIndent);
 
-//
-// Test if the given field is set on a message.
-//
+/// Test if the given field is set on a message.
 BOOL GPBMessageHasFieldNumberSet(GPBMessage *self, uint32_t fieldNumber);
+/// Test if the given field is set on a message.
 BOOL GPBMessageHasFieldSet(GPBMessage *self, GPBFieldDescriptor *field);
 
-//
-// Clear the given field of a message.
-//
+/// Clear the given field of a message.
 void GPBClearMessageField(GPBMessage *self, GPBFieldDescriptor *field);
 
 //%PDDM-EXPAND GPB_ACCESSORS()
@@ -68,60 +78,100 @@
 
 // Single Fields
 
+/// Gets the value of a bytes field.
 NSData *GPBGetMessageBytesField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a bytes field.
 void GPBSetMessageBytesField(GPBMessage *self, GPBFieldDescriptor *field, NSData *value);
 
+/// Gets the value of a string field.
 NSString *GPBGetMessageStringField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a string field.
 void GPBSetMessageStringField(GPBMessage *self, GPBFieldDescriptor *field, NSString *value);
 
+/// Gets the value of a message field.
 GPBMessage *GPBGetMessageMessageField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a message field.
 void GPBSetMessageMessageField(GPBMessage *self, GPBFieldDescriptor *field, GPBMessage *value);
 
+/// Gets the value of a group field.
 GPBMessage *GPBGetMessageGroupField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a group field.
 void GPBSetMessageGroupField(GPBMessage *self, GPBFieldDescriptor *field, GPBMessage *value);
 
+/// Gets the value of a bool field.
 BOOL GPBGetMessageBoolField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a bool field.
 void GPBSetMessageBoolField(GPBMessage *self, GPBFieldDescriptor *field, BOOL value);
 
+/// Gets the value of an int32 field.
 int32_t GPBGetMessageInt32Field(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of an int32 field.
 void GPBSetMessageInt32Field(GPBMessage *self, GPBFieldDescriptor *field, int32_t value);
 
+/// Gets the value of an uint32 field.
 uint32_t GPBGetMessageUInt32Field(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of an uint32 field.
 void GPBSetMessageUInt32Field(GPBMessage *self, GPBFieldDescriptor *field, uint32_t value);
 
+/// Gets the value of an int64 field.
 int64_t GPBGetMessageInt64Field(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of an int64 field.
 void GPBSetMessageInt64Field(GPBMessage *self, GPBFieldDescriptor *field, int64_t value);
 
+/// Gets the value of an uint64 field.
 uint64_t GPBGetMessageUInt64Field(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of an uint64 field.
 void GPBSetMessageUInt64Field(GPBMessage *self, GPBFieldDescriptor *field, uint64_t value);
 
+/// Gets the value of a float field.
 float GPBGetMessageFloatField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a float field.
 void GPBSetMessageFloatField(GPBMessage *self, GPBFieldDescriptor *field, float value);
 
+/// Gets the value of a double field.
 double GPBGetMessageDoubleField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a double field.
 void GPBSetMessageDoubleField(GPBMessage *self, GPBFieldDescriptor *field, double value);
 
-// Get/Set the given enum field of a message.  You can only Set values that are
-// members of the enum.  For proto3, when doing a Get, if the value isn't a
-// memeber of the enum, kGPBUnrecognizedEnumeratorValue will be returned. The
-// the functions with "Raw" in the will bypass all checks.
+/// Get the given enum field of a message. For proto3, if the value isn't a
+/// member of the enum, @c kGPBUnrecognizedEnumeratorValue will be returned.
+/// GPBGetMessageRawEnumField will bypass the check and return whatever value
+/// was set.
 int32_t GPBGetMessageEnumField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Set the given enum field of a message. You can only set values that are
+/// members of the enum.
 void GPBSetMessageEnumField(GPBMessage *self, GPBFieldDescriptor *field, int32_t value);
+/// Get the given enum field of a message. No check is done to ensure the value
+/// was defined in the enum.
 int32_t GPBGetMessageRawEnumField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Set the given enum field of a message. You can set the value to anything,
+/// even a value that is not a member of the enum.
 void GPBSetMessageRawEnumField(GPBMessage *self, GPBFieldDescriptor *field, int32_t value);
 
 // Repeated Fields
 
-// The object will/should be GPB*Array or NSMutableArray based on the field's
-// type.
+/// Gets the value of a repeated field.
+///
+/// The result will be @c GPB*Array or @c NSMutableArray based on the
+/// field's type.
 id GPBGetMessageRepeatedField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a repeated field.
+///
+/// The value should be @c GPB*Array or @c NSMutableArray based on the
+/// field's type.
 void GPBSetMessageRepeatedField(GPBMessage *self, GPBFieldDescriptor *field, id array);
 
 // Map Fields
 
-// The object will/should be GPB*Dictionary or NSMutableDictionary based on the
-// field's type.
+/// Gets the value of a map<> field.
+///
+/// The result will be @c GPB*Dictionary or @c NSMutableDictionary based on
+/// the field's type.
 id GPBGetMessageMapField(GPBMessage *self, GPBFieldDescriptor *field);
+/// Sets the value of a map<> field.
+///
+/// The object should be @c GPB*Dictionary or @c NSMutableDictionary based
+/// on the field's type.
 void GPBSetMessageMapField(GPBMessage *self, GPBFieldDescriptor *field, id dictionary);
 
 //%PDDM-EXPAND-END GPB_ACCESSORS()
@@ -144,44 +194,64 @@
 //%
 //%// Single Fields
 //%
-//%GPB_ACCESSOR_SINGLE_FULL(Bytes, NSData, *)
-//%GPB_ACCESSOR_SINGLE_FULL(String, NSString, *)
-//%GPB_ACCESSOR_SINGLE_FULL(Message, GPBMessage, *)
-//%GPB_ACCESSOR_SINGLE_FULL(Group, GPBMessage, *)
-//%GPB_ACCESSOR_SINGLE(Bool, BOOL)
-//%GPB_ACCESSOR_SINGLE(Int32, int32_t)
-//%GPB_ACCESSOR_SINGLE(UInt32, uint32_t)
-//%GPB_ACCESSOR_SINGLE(Int64, int64_t)
-//%GPB_ACCESSOR_SINGLE(UInt64, uint64_t)
-//%GPB_ACCESSOR_SINGLE(Float, float)
-//%GPB_ACCESSOR_SINGLE(Double, double)
-//%// Get/Set the given enum field of a message.  You can only Set values that are
-//%// members of the enum.  For proto3, when doing a Get, if the value isn't a
-//%// memeber of the enum, kGPBUnrecognizedEnumeratorValue will be returned. The
-//%// the functions with "Raw" in the will bypass all checks.
+//%GPB_ACCESSOR_SINGLE_FULL(Bytes, NSData, , *)
+//%GPB_ACCESSOR_SINGLE_FULL(String, NSString, , *)
+//%GPB_ACCESSOR_SINGLE_FULL(Message, GPBMessage, , *)
+//%GPB_ACCESSOR_SINGLE_FULL(Group, GPBMessage, , *)
+//%GPB_ACCESSOR_SINGLE(Bool, BOOL, )
+//%GPB_ACCESSOR_SINGLE(Int32, int32_t, n)
+//%GPB_ACCESSOR_SINGLE(UInt32, uint32_t, n)
+//%GPB_ACCESSOR_SINGLE(Int64, int64_t, n)
+//%GPB_ACCESSOR_SINGLE(UInt64, uint64_t, n)
+//%GPB_ACCESSOR_SINGLE(Float, float, )
+//%GPB_ACCESSOR_SINGLE(Double, double, )
+//%/// Get the given enum field of a message. For proto3, if the value isn't a
+//%/// member of the enum, @c kGPBUnrecognizedEnumeratorValue will be returned.
+//%/// GPBGetMessageRawEnumField will bypass the check and return whatever value
+//%/// was set.
 //%int32_t GPBGetMessageEnumField(GPBMessage *self, GPBFieldDescriptor *field);
+//%/// Set the given enum field of a message. You can only set values that are
+//%/// members of the enum.
 //%void GPBSetMessageEnumField(GPBMessage *self, GPBFieldDescriptor *field, int32_t value);
+//%/// Get the given enum field of a message. No check is done to ensure the value
+//%/// was defined in the enum.
 //%int32_t GPBGetMessageRawEnumField(GPBMessage *self, GPBFieldDescriptor *field);
+//%/// Set the given enum field of a message. You can set the value to anything,
+//%/// even a value that is not a member of the enum.
 //%void GPBSetMessageRawEnumField(GPBMessage *self, GPBFieldDescriptor *field, int32_t value);
 //%
 //%// Repeated Fields
 //%
-//%// The object will/should be GPB*Array or NSMutableArray based on the field's
-//%// type.
+//%/// Gets the value of a repeated field.
+//%///
+//%/// The result will be @c GPB*Array or @c NSMutableArray based on the
+//%/// field's type.
 //%id GPBGetMessageRepeatedField(GPBMessage *self, GPBFieldDescriptor *field);
+//%/// Sets the value of a repeated field.
+//%///
+//%/// The value should be @c GPB*Array or @c NSMutableArray based on the
+//%/// field's type.
 //%void GPBSetMessageRepeatedField(GPBMessage *self, GPBFieldDescriptor *field, id array);
 //%
 //%// Map Fields
 //%
-//%// The object will/should be GPB*Dictionary or NSMutableDictionary based on the
-//%// field's type.
+//%/// Gets the value of a map<> field.
+//%///
+//%/// The result will be @c GPB*Dictionary or @c NSMutableDictionary based on
+//%/// the field's type.
 //%id GPBGetMessageMapField(GPBMessage *self, GPBFieldDescriptor *field);
+//%/// Sets the value of a map<> field.
+//%///
+//%/// The object should be @c GPB*Dictionary or @c NSMutableDictionary based
+//%/// on the field's type.
 //%void GPBSetMessageMapField(GPBMessage *self, GPBFieldDescriptor *field, id dictionary);
 //%
 
-//%PDDM-DEFINE GPB_ACCESSOR_SINGLE(NAME, TYPE)
-//%GPB_ACCESSOR_SINGLE_FULL(NAME, TYPE, )
-//%PDDM-DEFINE GPB_ACCESSOR_SINGLE_FULL(NAME, TYPE, TisP)
+//%PDDM-DEFINE GPB_ACCESSOR_SINGLE(NAME, TYPE, AN)
+//%GPB_ACCESSOR_SINGLE_FULL(NAME, TYPE, AN, )
+//%PDDM-DEFINE GPB_ACCESSOR_SINGLE_FULL(NAME, TYPE, AN, TisP)
+//%/// Gets the value of a##AN NAME$L field.
 //%TYPE TisP##GPBGetMessage##NAME##Field(GPBMessage *self, GPBFieldDescriptor *field);
+//%/// Sets the value of a##AN NAME$L field.
 //%void GPBSetMessage##NAME##Field(GPBMessage *self, GPBFieldDescriptor *field, TYPE TisP##value);
 //%