doc: Clarify how to remove a QTextFrame

The documentation wrongfully recommended deleting QTextFrames
directly. This would cause a crash, since the destructor didn't
update the document's layout at all. The correct way is the same
as when removing other aspects of the document.

Task-number: QTBUG-53082
Change-Id: I64f0ad08f1d063626456fa51d03611871ce6aa45
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>