Unified duplicate warning in mod_proxy ProxyPass documentation.

Also changed terminology to be in sync with:
https://httpd.apache.org/docs/current/mod/directive-dict.html#Context

It was confusing in my opinion to read that ProxyPass wasn't supported
for the <Directory> context (since "directory" semantic is already
overloaded).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769637 13f79535-47bb-0310-9956-ffa450edef68