Improve mod_proxy_html doc

   - add some links and color highligh
   - remove some <var> (i.e. italic) around parameters that should be written unmodified (On|Off...)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1813997 13f79535-47bb-0310-9956-ffa450edef68