Document `ErrorLog syslog:facility' variant of error logging.

Requested by: Youichirou Koga <y-koga@jp.FreeBSD.org>
Performed by: Ralf S. Engelschall
PR: 3096


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@82204 13f79535-47bb-0310-9956-ffa450edef68