expand on ProxyPassReverse args

split up the two arguments into their own paragraphs
try to reinforce that the 2nd arg has to match the response
hedaer, and what the first one is used for.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836683 13f79535-47bb-0310-9956-ffa450edef68