docs: QPlatformWindow::screen() and QWindow::screen() can return null

After change f4b8697c40, it is normal
on X11 to have a null screen when all outputs have been disconnected
or turned off.

Change-Id: I97eeefd86d97701be50f0757fe5c53ca36d79aaa
Reviewed-by: Harri Porten <porten@froglogic.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/gui/kernel/qplatformwindow.cpp

@@ -79,7 +79,8 @@ QPlatformWindow *QPlatformWindow::parent() const
 }
 
 /*!
-    Returns the platform screen handle corresponding to this platform window.
+    Returns the platform screen handle corresponding to this platform window,
+    or null if the window is not associated with a screen.
 */
 QPlatformScreen *QPlatformWindow::screen() const
 {

src/gui/kernel/qwindow.cpp

@@ -1678,7 +1678,7 @@ bool QWindow::setMouseGrabEnabled(bool grab)
 }
 
 /*!
-    Returns the screen on which the window is shown.
+    Returns the screen on which the window is shown, or null if there is none.
 
     For child windows, this returns the screen of the corresponding top level window.