Adding a note to mod_proxy documentation to clarify DNS resolution with origin...

Adding a note to mod_proxy documentation to clarify DNS resolution with origin servers. Follow up from a question on users@. Backported from trunk.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1745343 13f79535-47bb-0310-9956-ffa450edef68