Fix default FEED_DOMAIN: None -> SITEURL.

This is similar to how `AUTHOR_REGEX_SUBSTITUTIONS` is described, for
instance.

Author: clockback

docs/settings.rst

@@ -971,8 +971,7 @@ the ``TAG_FEED_ATOM`` and ``TAG_FEED_RSS`` settings:
    absolute, it is highly recommended to define this (e.g.,
    "https://feeds.example.com"). If you have already explicitly defined SITEURL
    (see above) and want to use the same domain for your feeds, you can just
-   set:  ``FEED_DOMAIN = SITEURL``. The default is ``None``, which uses the
-   base URL "/".
+   set:  ``FEED_DOMAIN = SITEURL``. The default is `SITEURL`.
 
 .. data:: FEED_ATOM