make comment on invoke/sendmsg more clear

call.go

@@ -97,8 +97,9 @@ func sendRequest(ctx context.Context, codec Codec, compressor Compressor, callHd
 	return stream, nil
 }
 
-// Invoke is called by the generated code. It sends the RPC request on the
-// wire and returns after response is received.
+// Invoke sends the RPC request on the wire and returns after response is received.
+// Invoke is called by generated code. Also users can call Invoke directly when it
+// is really needed in their use cases.
 func Invoke(ctx context.Context, method string, args, reply interface{}, cc *ClientConn, opts ...CallOption) (err error) {
 	var c callInfo
 	for _, o := range opts {