Backport:

Overhaul the caching guide in an effort to clearly distinguish between
the mod_cache caching, the socache caching, and other caching we do, such
as mod_file_cache.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1210727 13f79535-47bb-0310-9956-ffa450edef68