Doc: Document asynchronous nature of QMediaPlayer::setMedia()

The function name can be interpreted to mean that it loads the media.
Users have made this mistake before:

http://comments.gmane.org/gmane.comp.lib.qt.user/9189
https://bugreports.qt-project.org/browse/QTBUG-28514

Change-Id: I9e9b36405d67d40db42b77d8eeb8f9d528ee16e2
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>