xdg-shell: Clarify set_fullscreen semantics

It was not explicitly specified (as it is in set/unset_maximize) that
the compositor will respond with a configure event when a client asks to
be fullscreened, and the meaning of the output parameter was somewhat
awkwardly described.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Mike Blumenkrantz <zmike@samsung.com>