Document the BC break of viewportSizeHint() in itemviews

Since users were not supposed to use this feature at all, just
live with the BC break, but document it. In order to prevent possible
problems in the future, introduce dummy overrides in QAbstractItemView
and QListView as well. Also, fix the visibility of the overrides
(it's protected, not public).

Task-number: QTBUG-34667
Change-Id: Ib4554ae5e1e7d3ce4ea8e8703e1fea9e2231edb0
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>