doc: update mode type for mkdir() functions

This commit updates the documentation for fs.mkdir(),
fs.mkdirSync(), and fsPromises.mkdir() to reflect the fact
that their mode option can be a string.

PR-URL: #31115
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/fs.md

@@ -2433,7 +2433,7 @@ changes:
 * `path` {string|Buffer|URL}
 * `options` {Object|integer}
   * `recursive` {boolean} **Default:** `false`
-  * `mode` {integer} Not supported on Windows. **Default:** `0o777`.
+  * `mode` {string|integer} Not supported on Windows. **Default:** `0o777`.
 * `callback` {Function}
   * `err` {Error}
 
@@ -2481,7 +2481,7 @@ changes:
 * `path` {string|Buffer|URL}
 * `options` {Object|integer}
   * `recursive` {boolean} **Default:** `false`
-  * `mode` {integer} Not supported on Windows. **Default:** `0o777`.
+  * `mode` {string|integer} Not supported on Windows. **Default:** `0o777`.
 
 Synchronously creates a directory. Returns `undefined`.
 This is the synchronous version of [`fs.mkdir()`][].
@@ -4793,7 +4793,7 @@ added: v10.0.0
 * `path` {string|Buffer|URL}
 * `options` {Object|integer}
   * `recursive` {boolean} **Default:** `false`
-  * `mode` {integer} Not supported on Windows. **Default:** `0o777`.
+  * `mode` {string|integer} Not supported on Windows. **Default:** `0o777`.
 * Returns: {Promise}
 
 Asynchronously creates a directory then resolves the `Promise` with no