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...)

r1813997 in trunk + some small modifications to synch with trunk

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1814012 13f79535-47bb-0310-9956-ffa450edef68