Provide a hint to those stuck with mod_mem_cache about coping with the per-process limitation.

Add detail about another deficiency relative to disk_cache [1 variant at a time].

PR 47783 
Reported By: Stoehr Sukachevin



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@814313 13f79535-47bb-0310-9956-ffa450edef68