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>