Allow the UserDir directive to accept a list of directories.

This matches what Apache 1.3 does.  Also add documentation for
this feature.

PR:	9299
Submitted by:	Jay Ball <jay@veggiespam.com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97001 13f79535-47bb-0310-9956-ffa450edef68