doc: improve callback params for fs.mkdir

fixes: nodejs#43015

Signed-off-by: Daeyeon Jeong daeyeon.dev@gmail.com

PR-URL: nodejs#43016
Fixes: nodejs#43015
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Zeyu "Alex" Yang <himself65@outlook.com>

Author: daeyeon

doc/api/fs.md

@@ -3007,6 +3007,8 @@ changes:
   * `mode` {string|integer} Not supported on Windows. **Default:** `0o777`.
 * `callback` {Function}
   * `err` {Error}
+  * `path` {string|undefined} Present only if a directory is created with
+    `recursive` set to `true`.
 
 Asynchronously creates a directory.