* mod_proxy_balancer: Document BALANCER_ROUTE_CHANGED environment variable.

Submitted by: Brian Rectanus <brectanu gmail.com>
Reviewed by: rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@453630 13f79535-47bb-0310-9956-ffa450edef68