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

Documentation/doc-guide/sphinx.rst

@@ -151,6 +151,13 @@ If the theme is not available, it will fall-back to the classic one.
 
 The Sphinx theme can be overridden by using the ``DOCS_THEME`` make variable.
 
+There is another make variable ``SPHINXDIRS``, which is useful when test
+building a subset of documentation.  For example, you can build documents
+under ``Documentation/doc-guide`` by running
+``make SPHINXDIRS=doc-guide htmldocs``.
+The documentation section of ``make help`` will show you the list of
+subdirectories you can specify.
+
 To remove the generated documentation, run ``make cleandocs``.
 
 .. [#ink] Having ``inkscape(1)`` from Inkscape (https://inkscape.org)