xdg-shell: Clarify that geometry doesn't automatically change

The spec says that

	When applied, the effective window geometry will be the set
	window geometry clamped to the bounding rectangle of the combined
	geometry of the surface of the xdg_surface and the associated
	subsurfaces.

Thus, a client cannot assume the geometry will adapt to any subsequent
changes to any conditions that constrained the geometry.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>