Document Qt.application.supportsMultipleWindows property.

The documentation was not added in
0df606e2ab8f3b60e1ad57dba245acf2e7810612.

Change-Id: I0a5802a66021e17d1280f3969981c9e8a62c8119
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>