doc: clarify how ClientAPI.xml is built
doc: clarify how ClientAPI.xml is built

I found the comment a bit confusing and it's quite hard to read. re-explain
with a simple step-by-step list

Signed-off-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>

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