nit: Documentation and formatting.

src/objective-c/GRPCClient/GRPCCall.h

@@ -55,6 +55,13 @@
 extern id const kGRPCHeadersKey;
 extern id const kGRPCTrailersKey;
 
+// The container of the request headers of an RPC conforms to this protocol, which is a subset of
+// NSMutableDictionary's interface. It will become a NSMutableDictionary later on.
+// The keys of this container are the header names, which per the HTTP standard are case-
+// insensitive. They are stored in lowercase (which is how HTTP/2 mandates them on the wire), and
+// can only consist of ASCII characters.
+// A header value is a NSString object (with only ASCII characters), unless the header name has the
+// suffix "-bin", in which case the value has to be a NSData object.
 @protocol GRPCRequestHeaders <NSObject>
 
 @property(nonatomic, readonly) NSUInteger count;

src/objective-c/GRPCClient/private/GRPCRequestHeaders.h

@@ -47,6 +47,6 @@
 - (void)setObject:(id)obj forKeyedSubscript:(NSString *)key;
 
 - (void)removeAllObjects;
-- (void)removeObjectForKey:(NSString *)aKey;
+- (void)removeObjectForKey:(NSString *)key;
 
-@end
+@end
\ No newline at end of file

src/objective-c/GRPCClient/private/GRPCRequestHeaders.m

@@ -117,4 +117,4 @@ static void CheckKeyValuePairIsValid(NSString *key, id value) {
 - (grpc_metadata *)grpc_metadataArray {
   return _proxy.grpc_metadataArray;
 }
-@end
+@end
\ No newline at end of file