Expand on mod_alias documentation.

Add a link from the glossary entry on regular expressions to the wikipedia
page that seems to be a good reference for the syntax of PCRE's flavor of
regular expressions.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@890436 13f79535-47bb-0310-9956-ffa450edef68