Bug 1394: Document how to do per host conn limits

Bug 1394: Document how to do per host conn limits

Since the max connections per host setting is computed and enforced
  in the master listening process before the fork, there is no easy
  way to get an accurate connection count once the Proxy Protocol
  negotiation has been done (i.e. in a child process, after the
  fork). Rather than try to use a shared mmap file using CAS in the
  children to manipulate it, we just advise of a crude version of
  max connections per IP be achieved by using ratelimit per_conn in
  the connect ACL.