Update the comment and checks around address/connection reusability

in mod_proxy.  

The checks and comment are a vestige of 2.0.x
mod_proxy_http where the backend proxy conn was stored in 
the frontend r->conn_config, but ap_proxy_determine_connection()
doesn't associate the frontend and backend conns like this.




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1227642 13f79535-47bb-0310-9956-ffa450edef68