Documentation should mention that QPA on 4.8 is experimental

In http://qt-project.org/wiki/DirectFBAndQt it is mentioned, that "QPA is experimental in Qt 4.8 and is the base technology of Qt5. ", but in http://qt-project.org/doc/qt-4.8/qt4-8-intro.html it says "QPA allows porting Qt to different windowing systems and devices easier. It provides a clean abstraction layer for porting QtGui to new window systems." so it should be mentioned there too, otherwise users may get the wrong impression.