utils: Reference some useful methods in wl_signal's doxygen
utils: Reference some useful methods in wl_signal's doxygen

This commit adds a bit more detail on the lifecycle of a signal.

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