ObjC: Document the exceptions on some of the writing apis.

objectivec/GPBMessage.h

diff --git a/objectivec/GPBMessage.h b/objectivec/GPBMessage.h
index 2c325ba..276740d 100644
--- a/objectivec/GPBMessage.h
+++ b/objectivec/GPBMessage.h
@@ -292,6 +292,9 @@
  * Writes out the message to the given coded output stream.
  *
  * @param output The coded output stream into which to write the message.
+ *
+ * @note This can raise the GPBCodedOutputStreamException_* exceptions.
+ *
  **/
 - (void)writeToCodedOutputStream:(GPBCodedOutputStream *)output;
 
@@ -299,6 +302,8 @@
  * Writes out the message to the given output stream.
  *
  * @param output The output stream into which to write the message.
+ *
+ * @note This can raise the GPBCodedOutputStreamException_* exceptions.
  **/
 - (void)writeToOutputStream:(NSOutputStream *)output;
 
@@ -307,6 +312,8 @@
  * the given output stream.
  *
  * @param output The coded output stream into which to write the message.
+ *
+ * @note This can raise the GPBCodedOutputStreamException_* exceptions.
  **/
 - (void)writeDelimitedToCodedOutputStream:(GPBCodedOutputStream *)output;
 
@@ -315,6 +322,8 @@
  * the given output stream.
  *
  * @param output The output stream into which to write the message.
+ *
+ * @note This can raise the GPBCodedOutputStreamException_* exceptions.
  **/
 - (void)writeDelimitedToOutputStream:(NSOutputStream *)output;