Improve doc on how to implement the QIODevice based stream playback

void QMediaPlayer::setMedia ( const QMediaContent & media, QIODevice * stream = 0 );
void VideoPlayer::play ( const Phonon::MediaSource & source )

Both QMediaPlayer and Phonon is expected to support playing QIODevice based streams, but there is no description or guide on how to implement the QIODevice based stream playback using current APIs.

Priority 1: doc improvements for application developer
Priority 2: doc improvements for backend implementor