Undocumented behavior on QMediaPlayer setPlaybackRate for higher values

When setting the QMediaPlayer property to values greater than 4.0, the value is set to 1.0 instead, and an adequate playbackRateChanged signal is emitted.

The [QMediaPlayer playbackRate](http://doc.qt.io/qt-5/qmediaplayer.html#playbackRate-prop) documentation does not state anything about potentially unexpected values being set / playbackRate being reset to 1.0.

It documents:
> It is framework defined as to the status and quality of audio and video while fast forwarding or rewinding.

While I do not know what "framework defined" even means (Qt is the framework here, I guess not?), I would expect setting the playback speed to 5.0 would actually set it to 5.0, potentially leading to playback issues or at least setting it to the maximum of 4.0.

I would expect the documentation to document this behavior, or if this is specific to an underlying library ("framework defined"?) to be pointed there and the potential of the value changing to other values be mentioned. If on the other hand this is a bug rather than incomplete documentation, other steps would be necessary I guess.