wayland-server: Improve wording for wl_signal_get's doc
wayland-server: Improve wording for wl_signal_get's doc

The old description was a bit vague; this commit hopefully improves
describing what is returned.

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