[PATCH 2/5] docs/doc-guide: Mention make variable SPHINXDIRS

[Akira Yokosawa <akiyks@gmail.com>]

SPHINXDIRS is useful when you want test builds of 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>
---
This change is inspired from correspondence with Maciej [1].

[1]: https://lore.kernel.org/r/f4d40da6-756b-9e75-b867-cc9eedc4b232@gmail.com

--
 Documentation/doc-guide/sphinx.rst | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/doc-guide/sphinx.rst b/Documentation/doc-guide/sphinx.rst
index edc4fa023986..efcccab68286 100644
--- a/Documentation/doc-guide/sphinx.rst
+++ b/Documentation/doc-guide/sphinx.rst
@@ -151,6 +151,10 @@ 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.  Again, see the documentation section
+of ``make help`` for the details.
+
 To remove the generated documentation, run ``make cleandocs``.
 
 .. [#ink] Having ``inkscape(1)`` from Inkscape (https://inkscape.org)
-- 
2.25.1