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>