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.

1 files changed, 32 insertions, 15 deletions

diff --git a/objectivec/GPBUnknownFieldSet.h b/objectivec/GPBUnknownFieldSet.h
index cf612993..1b5f24f3 100644
--- a/objectivec/GPBUnknownFieldSet.h
+++ b/objectivec/GPBUnknownFieldSet.h
@@ -34,31 +34,48 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
-/// A collection of unknown fields.
+/**
+ * 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.
+ **/
 @interface GPBUnknownFieldSet : NSObject<NSCopying>
 
-/// Tests to see if the given field number has a value.
-///
-/// @param number The field number to check.
-///
-/// @return YES if there is an unknown field for the given field number.
+/**
+ * Tests to see if the given field number has a value.
+ *
+ * @param number The field number to check.
+ *
+ * @return YES if there is an unknown field for the given field number.
+ **/
 - (BOOL)hasField:(int32_t)number;
 
-/// Fetches the @c GPBUnknownField for the given field number.
-///
-/// @param number The field number to look up.
-///
-/// @return The @c GPBUnknownField or nil.
+/**
+ * Fetches the GPBUnknownField for the given field number.
+ *
+ * @param number The field number to look up.
+ *
+ * @return The GPBUnknownField or nil if none found.
+ **/
 - (nullable GPBUnknownField *)getField:(int32_t)number;
 
-/// Returns the number of fields in this set.
+/**
+ * @return The number of fields in this set.
+ **/
 - (NSUInteger)countOfFields;
 
-/// Adds the given field to the set.
+/**
+ * Adds the given field to the set.
+ *
+ * @param field The field to add to the set.
+ **/
 - (void)addField:(GPBUnknownField *)field;
 
-/// Returns an NSArray of the @c GPBUnknownFields sorted by the field numbers.
-- (NSArray<GPBUnknownField*> *)sortedFields;
+/**
+ * @return An array of the GPBUnknownFields sorted by the field numbers.
+ **/
+- (NSArray<GPBUnknownField *> *)sortedFields;
 
 @end