Polish and add comments

src/objective-c/GRPCClient/GRPCCall.h

@@ -269,6 +269,12 @@ extern NSString *const kGRPCTrailersKey;
  */
 - (void)finish;
 
+/**
+ * Tell gRPC to receive the next N gRPC message from gRPC core.
+ *
+ * This method should only be used when flow control is enabled. When flow control is not enabled,
+ * this method is a no-op.
+ */
 - (void)receiveNextMessages:(NSUInteger)numberOfMessages;
 
 /**

src/objective-c/ProtoRPC/ProtoRPC.h

@@ -138,16 +138,22 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)finish;
 
 /**
- * Tell gRPC to receive the next message. If flow control is enabled, the messages received from the
- * server are buffered in gRPC until the user want to receive the next message. If flow control is
- * not enabled, messages will be automatically received after the previous one is delivered.
+ * Tell gRPC to receive another message.
+ *
+ * This method should only be used when flow control is enabled. If flow control is enabled, gRPC
+ * will only receive additional messages after the user indicates so by using either
+ * receiveNextMessage: or receiveNextMessages: methods. If flow control is not enabled, messages
+ * will be automatically received after the previous one is delivered.
  */
 - (void)receiveNextMessage;
 
 /**
- * Tell gRPC to receive the next N message. If flow control is enabled, the messages received from
- * the server are buffered in gRPC until the user want to receive the next message. If flow control
- * is not enabled, messages will be automatically received after the previous one is delivered.
+ * Tell gRPC to receive another N message.
+ *
+ * This method should only be used when flow control is enabled. If flow control is enabled, the
+ * messages received from the server are buffered in gRPC until the user want to receive the next
+ * message. If flow control is not enabled, messages will be automatically received after the
+ * previous one is delivered.
  */
 - (void)receiveNextMessages:(NSUInteger)numberOfMessages;