doc: improve callback params for fs.mkdir

fixes: nodejs/node#43015

Signed-off-by: Daeyeon Jeong [email protected]

PR-URL: nodejs/node#43016
Fixes: nodejs/node#43015
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>
Reviewed-By: Akhil Marsonya <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Zeyu "Alex" Yang <[email protected]>

Author: daeyeon

doc/api/fs.md

@@ -2822,6 +2822,8 @@ 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.