Add a QGrpcClientStream::writesDone() usage in the doc

link: https://doc.qt.io/qt-6/qtgrpc-service-client-methods.html#using-the-client-streams-in-qt-grpc

This article explains how a Qt grpc client can send and recieve messages.

There is a section(the link above) in which how a client can send a stream to a server. There, it uses QTimer to send messages endlessly with QGrpcClientStream::writeMessage().
I assume the server somehow decides when it's gotten "enough" with its application logic.
I think it'd be informative to also add a case where a client calls QGrpcClientStream::writesDone() to achieve a half-closing.