[ObjC] Remove some stale comments about proto3 and unknown fields.

PiperOrigin-RevId: 488363764

objectivec/GPBMessage.h

@@ -90,15 +90,13 @@ CF_EXTERN_C_END
  * The set of unknown fields for this message.
  *
  * Only messages from proto files declared with "proto2" syntax support unknown
- * fields. For "proto3" syntax, any unknown fields found while parsing are
- * dropped.
+ * fields.
  **/
 @property(nonatomic, copy, nullable) GPBUnknownFieldSet *unknownFields;
 
 /**
  * Whether the message, along with all submessages, have the required fields
- * set. This is only applicable for files declared with "proto2" syntax, as
- * there are no required fields for "proto3" syntax.
+ * set.
  **/
 @property(nonatomic, readonly, getter=isInitialized) BOOL initialized;
 

objectivec/GPBUnknownFieldSet.h

@@ -37,8 +37,7 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * A collection of unknown fields. Fields parsed from the binary representation
  * of a message that are unknown end up in an instance of this set. This only
- * applies for files declared with the "proto2" syntax. Files declared with the
- * "proto3" syntax discard the unknown values.
+ * applies for files declared with the "proto2" syntax.
  **/
 @interface GPBUnknownFieldSet : NSObject <NSCopying>