Add link to description of how files with multiple extensions are

handled. At the same time update the descriptions of the directives
to make the descriptions in the authoritative file (mod/mod_mime.html).

PR: 3151


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@82405 13f79535-47bb-0310-9956-ffa450edef68