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>

src/gui/text/qtextobject.cpp

@@ -412,9 +412,12 @@ QTextFrame::QTextFrame(QTextDocument *doc)
 {
 }
 
-// ### DOC: What does this do to child frames?
 /*!
-    Destroys the frame, and removes it from the document's layout.
+    Destroys the text frame.
+
+    \warning Text frames are owned by the document, so you should
+    never destroy them yourself. In order to remove a frame from
+    its document, remove its contents using a \c QTextCursor.
 */
 QTextFrame::~QTextFrame()
 {