Add the "How filters work" section. Cut 'n paste job from Ryans

email <022501c1c529$f63a9550$7f00000a@KOJ>, needs formatting.

Submitted by: Ryan Bloom


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93839 13f79535-47bb-0310-9956-ffa450edef68