Further sync docs with trunk.

I haven't backported docs for the cache-invalidate
env var, since although it is present in 2.4.x
the functionality around it was removed for future
definition, so I guess it is not really useful
right now.

Please correct me if tha is wrong and we should
add the cache-invalidate to the 2.4.x docs
like we have right now in trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1238800 13f79535-47bb-0310-9956-ffa450edef68