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/branches/APACHE_2_0_BRANCH@102296 13f79535-47bb-0310-9956-ffa450edef68