try to make the mod_filter docs a little more approachable

 - more consistently refer to the filters declared via the harness as
   "smart filters"
 - reinforce that the providers are traditional filters.
 - drop some info about the optional type parameter
 - elaborate on examples



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1829713 13f79535-47bb-0310-9956-ffa450edef68