Update the documentation of QScreen::name()

Task-number: QTBUG-109569
Change-Id: I24ed586e9094e7cd2277c6289ffbd4210dd6e430
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 9af5a70c55c9ec85113303fb48af405304335d1a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qscreen.cpp

@@ -150,6 +150,10 @@ QPlatformScreen *QScreen::handle() const
 
   For example, on X11 these correspond to the XRandr screen names,
   typically "VGA1", "HDMI1", etc.
+
+  \note The user presentable string is not guaranteed to match the
+  result of any native APIs, and should not be used to uniquely identify
+  a screen.
 */
 QString QScreen::name() const
 {