Merge r823536, r823563 from trunk:

mod_cache: add Cache-control: s-maxage to cacheability decisions per RFC 2616.

Update doc to match change to mod_cache to include s-maxage in
some cacheability decisions.

Reviewed by: poirier


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@831031 13f79535-47bb-0310-9956-ffa450edef68