Migrating documentation of the ObjectiveC runtime code to appledoc. (#1867)

Work for #1866 

Migrates all the public class docs over to appledoc format.  While Xcode is fine with blank lines in `///` comments, appledoc (used by cocoadocs) isn't and was leaving a bunch of info off the doc pages.

The generator still needs to be updated to do this also; that will be a follow up CL.

objectivec/GPBRootObject.h

diff --git a/objectivec/GPBRootObject.h b/objectivec/GPBRootObject.h
index c05b5c6..d2e2aeb 100644
--- a/objectivec/GPBRootObject.h
+++ b/objectivec/GPBRootObject.h
@@ -34,12 +34,17 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-/// Every generated proto file defines a local "Root" class that exposes a
-/// @c GPBExtensionRegistry for all the extensions defined by that file and
-/// the files it depends on.
+/**
+ * Every generated proto file defines a local "Root" class that exposes a
+ * GPBExtensionRegistry for all the extensions defined by that file and
+ * the files it depends on.
+ **/
 @interface GPBRootObject : NSObject
 
-/// An extension registry for the given file and all the files it depends on.
+/**
+ * @return An extension registry for the given file and all the files it depends
+ * on.
+ **/
 + (GPBExtensionRegistry *)extensionRegistry;
 
 @end