protocol: improve sub-surface spec wording
protocol: improve sub-surface spec wording

Mention, that sub-surfaces are not clipped to the parent.
Be more accurate on surface commit vs. apply state.
Mention the initial stacking order.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

giucam/weston,mchalupa/weston,Fantu/compositor-spice,sir-murray/weston,Fantu/compositor-spice,sir-murray/weston,krezovic/weston,mchalupa/weston,eyolfson/weston,xorgy/weston,jonnylamb/weston,kwm81/weston,giucam/weston,xorgy/weston,kwm81/weston,Gnurou/weston,jonnylamb/weston,udoprog/weston,Tarnyko/weston-xdg_surface_present,Gnurou/weston,Tarnyko/weston-xdg_surface_present,udoprog/weston,Fantu/compositor-spice,krezovic/weston,eyolfson/weston