Backport:

Create initial documentation for mod_socache_shmcb, mod_socache_dc,
mod_socache_dbm and mod_socache_memcache.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1210956 13f79535-47bb-0310-9956-ffa450edef68