Clarify some mod_proxy concepts in the docs

Clarify some mod_proxy concepts in the docs

I have recently got some basic mod-proxy concepts
and these simple info would have helped me a lot:

- connection pooling == reuse. My brain did not pick
  up this simple nomenclature at first read for some
  reason, probably because we have parameters called
  "*reuse*" and not "*pooling*". I find the term
  "reuse" clearer than "pooling", but this is
  debatable :)
- the concept of "Worker" needs to be read before
  the ProxyPass section (that people probably read
  directly after checking a search engine) to fully
  understand its power.
- the fact that a worker is related to a single process
  (and not shared as a Balancer) is a really important
  concept that I tried to highlight when talking
  about maxconns (question popped up a lot of times
  from users).

Hope that my understanding is correct, feedback is welcome.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1803171 13f79535-47bb-0310-9956-ffa450edef68