docs: Add details about surfaces
docs: Add details about surfaces

Mention some of the characteristic differences to X:
no global positions, no access to foreign windows.

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