doc: Slight tweaks to wl_listener
doc: Slight tweaks to wl_listener

Prefer \comment over // in code blocks for consistency's sake and keep
variable definitions separated by a line from the rest of the body.

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