fs: add explicit note about undefined path when recursive

PR-URL: #37010
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: eps1lon

Diff for: doc/api/fs.md

@@ -2608,6 +2608,8 @@ Asynchronously creates a directory.
 
 The callback is given a possible exception and, if `recursive` is `true`, the
 first directory path created, `(err, [path])`.
+`path` can still be `undefined` when `recursive` is `true`, if no directory was
+created.
 
 The optional `options` argument can be an integer specifying `mode` (permission
 and sticky bits), or an object with a `mode` property and a `recursive`