protocol: Fix documentation for the parameter of wl_callback
protocol: Fix documentation for the parameter of wl_callback

The parameter here is an opaque integer, rather than the event serial.
The "frame" callback uses this to pass the current time of day in
milliseconds.

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