Move the backend specific documentation into a nicer place.

We really don't need to bore the 99.9% of people who won't care.  And
it's certainly not more important than camera.

Reviewed-by: Derick Hawcroft
(cherry picked from commit 51e3a1bc45ffc2688fcd3949216aedda4c41bf81)

Change-Id: I8add47e42c2c06bf5e16f406604a19531af6901e
Reviewed-on: http://codereview.qt-project.org/5501
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimediakit/qmediaserviceprovider.cpp

@@ -85,7 +85,7 @@ public:
     \brief The QMediaServiceProviderHint class describes what is required of a QMediaService.
 
     \inmodule QtMultimediaKit
-    \ingroup multimedia
+    \ingroup multimedia-serv
     \since 1.0
 
     The QMediaServiceProvider class uses hints to select an appropriate media service.