- Add missing documentation for ProxyHTMLMeta directive.

- Tidy up mod_proxy_html docs
  PR 54269

- Fix validation error.

Backports of r1311468, r1420120 and r1420685 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1420728 13f79535-47bb-0310-9956-ffa450edef68