Document incompatible API change which happened

in 2.4.7. The change affects third party modules
using the mod_proxy shared memory structure 
proxy_worker_shared. Examples are third party
balancer lbmethod modules.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1738184 13f79535-47bb-0310-9956-ffa450edef68