Flush out the rewrite docs a little. In particular:

- rename flags to rewrite_flags for consistency
- note that flags is not comprehensive
- link up flags to the index page
- change the seealso links on the index page to be other useful
  links rather than a repeat of the links elsewhere on the page
- add some meat to the intro doc -- some sections still need more content



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@636374 13f79535-47bb-0310-9956-ffa450edef68