Update docs and comment: the unique id is now 24 characters, not 19

Submitted by: Takashi Sato <takashi lans-tv com>, Stefan Fritsch
PR: 36269


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@981084 13f79535-47bb-0310-9956-ffa450edef68