docs: update sphinx.rst to reflect the default theme change

We don't default to Read The Docs anymore; update the docs to match.

Acked-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/doc-guide/sphinx.rst

···
 format-specific subdirectories under ``Documentation/output``.
 
 To generate documentation, Sphinx (``sphinx-build``) must obviously be
-installed. For prettier HTML output, the Read the Docs Sphinx theme
-(``sphinx_rtd_theme``) is used if available. For PDF output you'll also need
-``XeLaTeX`` and ``convert(1)`` from ImageMagick
-(https://www.imagemagick.org).\ [#ink]_
-All of these are widely available and packaged in distributions.
+installed.  For PDF output you'll also need ``XeLaTeX`` and ``convert(1)``
+from ImageMagick (https://www.imagemagick.org).\ [#ink]_ All of these are
+widely available and packaged in distributions.
 
 To pass extra options to Sphinx, you can use the ``SPHINXOPTS`` make
 variable. For example, use ``make SPHINXOPTS=-v htmldocs`` to get more verbose
···
 It is also possible to pass an extra DOCS_CSS overlay file, in order to customize
 the html layout, by using the ``DOCS_CSS`` make variable.
 
-By default, the build will try to use the Read the Docs sphinx theme:
-
-    https://github.com/readthedocs/sphinx_rtd_theme
-
-If the theme is not available, it will fall-back to the classic one.
-
+By default, the "Alabaster" theme is used to build the HTML documentation;
+this theme is bundled with Sphinx and need not be installed separately.
 The Sphinx theme can be overridden by using the ``DOCS_THEME`` make variable.
 
 There is another make variable ``SPHINXDIRS``, which is useful when test