Expand mod_alias documentation.

Add link from glossary entry for regular expressions to the wikipedia entry,
which seems to be a good reference for PCRE's flavor of regular expression syntax.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@890440 13f79535-47bb-0310-9956-ffa450edef68