update worker doc to mention that

. a potentially large number of child processes can be in
  termination state

. ServerLimit/ThreadLimit should be coded before other worker
  MPM directives which control processes/threads


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@102295 13f79535-47bb-0310-9956-ffa450edef68