Document the X11 DISPLAY variable in the -display option docs

Change-Id: I20412616c91d187e2befc39134a480369347b8cb
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>

src/gui/kernel/qguiapplication.cpp

@@ -509,6 +509,8 @@ static QWindowGeometrySpecification windowGeometrySpecification;
 
     \list
         \li \c {-display} \e {hostname:screen_number}, switches displays on X11.
+
+             Overrides the \c DISPLAY environment variable.
         \li \c {-geometry} \e geometry, same as \c {-qwindowgeometry}.
     \endlist