QPainterStateGuard: document that restore() asserts when already at 0

Address comment from header review, amends
9ecf47a8a8d11227ecf192246d7df7c2c4dc9105.

Task-number: QTBUG-132090
Pick-to: 6.9
Change-Id: Ie6eed6f83fcdde0f90514b6016a65905505073e0
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/painting/qpainterstateguard.cpp

@@ -50,6 +50,8 @@ QT_BEGIN_NAMESPACE
 /*!
     \fn void QPainterStateGuard::restore()
     Calls QPainter::restore() if the internal save/restore counter is greater than zero.
+
+    \note This function asserts in debug builds if the counter has already reached zero.
 */
 
 QT_END_NAMESPACE