Update the proxy module docs to reflect current reality.

Add in the flush settings, which are part of 2.2.x and
reorder params to be in abc order (to make 'em easier
to find)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@432350 13f79535-47bb-0310-9956-ffa450edef68