compositor: add a comment about wlsc_surface::output
compositor: add a comment about wlsc_surface::output

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

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