This changes the description for URL property as it doesn't
support GraphicObject URL (vnd.sun.star.GraphicObject) anymore
so not a direct API change, but it is a significant functional
change.

Change-Id: Ie440d2140274243733a18ebc18089d2daa820c74
Reviewed-on: https://gerrit.libreoffice.org/53243Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>