compositor: Add missing help text
compositor: Add missing help text

Help messages were missing for some command line options.

Signed-off-by: Derek Foreman <2f97e0f0bc172406aab8ccd42dab54c95bab8759@osg.samsung.com>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

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