wayland-server: document WL_HIDE_DEPRECATED

Add some inline information, what the macro is used for, why it came to
be and what we shouldn't do if we consider further deprecation in the future
deprecation.

Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>