Make the warning in QBackingStore::endPaint() a little more helpful

Amends 2aa9908e24

Change-Id: I2883ca27b06b2b414b4991b2dab3f84100b4c853
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/painting/qbackingstore.cpp

@@ -186,7 +186,7 @@ QPaintDevice *QBackingStore::paintDevice()
 void QBackingStore::endPaint()
 {
     if (paintDevice()->paintingActive())
-        qWarning() << "QBackingStore::endPaint() called with active painter on backingstore paint device";
+        qWarning("QBackingStore::endPaint() called with active painter; did you forget to destroy it or call QPainter::end() on it?");
 
     handle()->endPaint();
 }