Incomplete Android service documentation

In Qt 5.10 a new class, QAndroidService was added. This class seems to be mandatory to use instead of a normal QCore/Gui/Application (See QTBUG-67298). Thus the documentation for android services (Meaningly this page: https://doc.qt.io/qt-5.10/androidservices.html) should be updated to state that this class must be used in order to create a working service.