xdg-shell: Clarify when application-specific metadata should be available to compositors
This is a follow-up from the discussion at [1], about application specific metadata and the fact compositors can only assume that they're present/set up after the initial commit. [1] wayland/weston#450 Suggested-by: Quentin Glidic <sardemff7+git@sardemff7.net> Signed-off-by: Marius Vlad <marius.vlad@collabora.com>