mod_cache: Clarify which scenario we have present, quick handler on and CACHE

filter present, which is an error, or the CACHE filter being present when the
cache is bypassed, which is business as usual.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1126575 13f79535-47bb-0310-9956-ffa450edef68