Make OS incompatibility issues more prominent in the docs

Qt attempts to support a wide range of technologies on multiple platforms in a consistent manner. However, owing to OS limitations and/or Qt development delays, that support is often not complete. While this is understandable, the documentation on these incompatibilities is incomplete and difficult to find.

Example 1: QAudioProbe (http://doc.qt.io/qt-5/qaudioprobe.html) is not supported on OSX but nowhere in the docs for that class does it state this. I wasted a day on this (and so did this person, apparently: http://stackoverflow.com/questions/29708986/qt-using-qaudioprobe-with-qmediaplayer) and it was only through the link provided in that answer that I discovered it (http://wiki.qt.io/Qt_5.5.0_Multimedia_Backends).

Example 2: Bluetooth and Bluetooth Examples
Again, incompatibilities and examples not working as a result.

There are many other examples.

Suggestion: In the header block of the documentation for each class, either state the currently supported (or unsupported) operating systems or link to a page which reports compatibility and issues for the feature (e.g. QAudioProbe > Multimedia, Bluetooth > Networking, etc.)