Document how to save an application using QUndoStack

Change-Id: Ic61374d5b6ce395dfe8d0b30813ea437b5e16e9d
Task-number: QTBUG-52497
Reviewed-by: Topi Reiniö <topi.reinio@theqtcompany.com>

src/widgets/util/qundostack.cpp

@@ -623,6 +623,8 @@ void QUndoStack::push(QUndoCommand *cmd)
     Marks the stack as clean and emits cleanChanged() if the stack was
     not already clean.
 
+    This is typically called when a document is saved, for example.
+
     Whenever the stack returns to this state through the use of undo/redo
     commands, it emits the signal cleanChanged(). This signal is also
     emitted when the stack leaves the clean state.