Document how to use QVideoSink::rhi() function

The QVideoSink::rhi() function is hard to use correctly, especially if the QVideoSink is managed by Qt. In this case, the QVideoSink's QRhi instance may be deleted by the framework, and the pointer returned by QVideoSink::rhi() may dangle.

The function QVideoSink::rhi() should be documented in a way that makes it easier to use.