update doc for connection pools

* reduce duplication between initial intro and parameter table
* explain that pools are per-process, and limits are not
  coordinated server-wide
* minor clarifications & readability tweaks


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@956059 13f79535-47bb-0310-9956-ffa450edef68