Rewrites the 'API Phases' section to give a brief intro to what an API

Phase is, and how mod_rewrite handles rewrite rules in two different
phases. Removes some of the condescending tone. References
https://issues.apache.org/bugzilla/show_bug.cgi?id=30021


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1180925 13f79535-47bb-0310-9956-ffa450edef68