protocol: Clarify semantics of sub-surface placement requests
protocol: Clarify semantics of sub-surface placement requests

Clarify some semantics of wl_subsurface.place_below and
wl_subsurface.place_below that were not specified.

Signed-off-by: Jonas Ådahl <67dcd8a17e758192e0cbb577ce1d62cc7da6cf49@gmail.com>
Reviewed-by: Jasper St. Pierre <5d8f4442e5d567aed616eebcfe54d707f2ec5349@mecheye.net>

sir-murray/wayland,eyolfson/wayland,tulcod/wayland,bitfactor/wayland,tulcod/wayland,bitfactor/wayland,abooij/wayland,bitfactor/wayland,mchalupa/wayland,mchalupa/wayland,eyolfson/wayland,mchalupa/wayland,sir-murray/wayland,sir-murray/wayland,eyolfson/wayland,tulcod/wayland,abooij/wayland,abooij/wayland