doc: standardize on sentence case for headers

Previously, our documentation headers were a mixture of title case,
sentence case, and things that were neither. For technical
documentation, the _de facto_ standard seems to be sentence case. (See
refs below.) So let's standardize on that. This commit follows a
commit implementing this standard. This commit adds it to the style
guide.

Refs: https://developers.google.com/style/capitalization#capitalization-in-titles-and-headings
Refs: https://docs.microsoft.com/en-us/style-guide/capitalization

PR-URL: #33889
Backport-PR-URL: #33964
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/guides/doc-style-guide.md

@@ -62,6 +62,9 @@
   * NOT OK: It is important to note that, in all cases, the return value will be
     a string regardless.
   <!-- lint enable prohibited-strings remark-lint-->
+* For headings, use sentence case, not title case.
+  * OK: _## Everybody to the limit_
+  * NOT OK: _## Everybody To The Limit_
 
 See also API documentation structure overview in [doctools README][].