Add a new <since> tag to ease writing compatibility notes.

Add a new <since> tag to ease writing compatibility notes.

This is much less verbose and will make wording more consistent in the generated html files.

It is declared in synopsis.xsl because its main use should be here, but it is usable anywhere.

Only the French translation is provided.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1919560 13f79535-47bb-0310-9956-ffa450edef68