libweston: improve weston_output_disable() comments

libweston: improve weston_output_disable() comments

Reorder some paragraphs to be more logically ordered. Rewrite the
description of the backend-specific disable function to explain the
semantics instead of the mechanics. Remove the paragraph about
pending_output_list as unnecessary details.

Add a big fat comment on why we call output->disable() always instead of
only for actually enabled outputs.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>