Add goroutine safety doc on stream (#1313)

stream.go

@@ -58,11 +58,17 @@ type Stream interface {
 	// side. On server side, it simply returns the error to the caller.
 	// SendMsg is called by generated code. Also Users can call SendMsg
 	// directly when it is really needed in their use cases.
+	// It's safe to have a goroutine calling SendMsg and another goroutine calling
+	// recvMsg on the same stream at the same time.
+	// But it is not safe to call SendMsg on the same stream in different goroutines.
 	SendMsg(m interface{}) error
 	// RecvMsg blocks until it receives a message or the stream is
 	// done. On client side, it returns io.EOF when the stream is done. On
 	// any other error, it aborts the stream and returns an RPC status. On
 	// server side, it simply returns the error to the caller.
+	// It's safe to have a goroutine calling SendMsg and another goroutine calling
+	// recvMsg on the same stream at the same time.
+	// But it is not safe to call RecvMsg on the same stream in different goroutines.
 	RecvMsg(m interface{}) error
 }