output-filters.xml: add a use case study

While working on http://svn.apache.org/r1833875
I learned a ton of non trivial things about
the output filter chain and how to design a filter,
and I thought to report them as a separate section
of the output filters doc page. Feedback is welcome
for inaccuracies or wrong understanding :)



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1834466 13f79535-47bb-0310-9956-ffa450edef68