doc: Warn against using the screen's refresh rate for driving animations

Pick-to: 6.2 6.3
Change-Id: Id26aae76792a4109b2ea6cca3db69ab505495d65
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/kernel/qscreen.cpp

@@ -547,6 +547,11 @@ Qt::ScreenOrientation QScreen::orientation() const
 /*!
   \property QScreen::refreshRate
   \brief the approximate vertical refresh rate of the screen in Hz
+
+  \warning Avoid using the screen's refresh rate to drive animations
+  via a timer such as QTimer. Instead use QWindow::requestUpdate().
+
+  \sa QWindow::requestUpdate()
 */
 qreal QScreen::refreshRate() const
 {