stable/presentation-time: reorganize clock_id documentation

Move compositor implementors' guidelines to the end. Recombine the
affected paragraphs.

No changes to the wording are made.

Suggested-by: Bryce Harrington <bryce@osg.samsung.com>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>