doc: mark QWindow::requestUpdate() as \since 5.5

Change-Id: I322e2e93edc4bdb6582c7614b9a8552221317553
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>

src/gui/kernel/qwindow.cpp

@@ -2100,6 +2100,8 @@ void QWindowPrivate::deliverUpdateRequest()
     be handled by the base class. For example, the default implementation of
     this function relies on QEvent::Timer events. Filtering them away would
     therefore break the delivery of the update events.
+
+    \since 5.5
 */
 void QWindow::requestUpdate()
 {