xdg-shell: Document error conditions when popup and surface getters

Document that a wl_surface can only be assigned either a xdg_popup or
xdg_surface once and that if the client still stries to do that an error
is raised.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Pekka Paalanen <pekka.paalanen@collabora.co.uk>
1 file changed