Document get_filenames().

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5317 2fea6a4d-a838-0410-917b-93a53c48e9d2
Moved get_filenames() to txplib_misc.php.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5318 2fea6a4d-a838-0410-917b-93a53c48e9d2