QGraphicsItem: document that setOpacity(0) almost makes the item invisible

This particular "feature" leaked into QtQuick1 back in the day. Still
it was never documented in QGraphicsView.

Task-number: QTBUG-18267
Change-Id: Ib5fb446015176f6e9b4095f6ec9030258cbd1ad4
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>