Because that probably isn't where people will look for it.
Thanks to Noah Lavine for the idea.
* doc/ref/api-regex.texi (Regular Expressions): New file, containing
the regex doc (promoted one level) that used to be in api-data.texi.
* doc/ref/guile.texi (API Reference): Include new file, and add menu
entry for the new section.
* THANKS: Add Noah.
