docs/doc-guide: Mention make variable SPHINXDIRS

SPHINXDIRS is useful when you want to test build only those
documents affected by your changes.

Mention it in the "Sphinx Build" section.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/50d6f0bc-030d-9529-0665-e2b3e7c792d8@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>