xdg-shell: move maximized state definition together

The xdg-shell documentation had part of the maximized state render
implications in the `set_maximized` request documentation, not the
actual state.
This moves the relevant lines into the state description.

Signed-off-by: Markus Ongyerth <wl@ongy.net>
Reviewed-by: Mike Blumenkrantz <zmike@osg.samsung.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>