Updated the perf-tuning documentation

Removed some out of date references and re-wrote
some sections. Added also a banner at the top of
the page to warn the users about stale content.
The next step is to improve the accept() related
documentation introducing the latest changes made
for event (and how awesome event is).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769877 13f79535-47bb-0310-9956-ffa450edef68