Doc: Update description of QAbstractItemModel::supportedDragActions()

Update documentation for the QAbstractItemModel::supportedDragActions()
(virtual since 5.0) and the obsoleted setter function.

Task-number: QTBUG-32410
Change-Id: I4f77601bca63e5f782ade1f577104500f541bbb1
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>