docs: add docs

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -646,6 +646,10 @@ You must set the error message for the closure rule.
 When you specify the error message, set the array key for the closure rule.
 In the above code, the ``required`` rule has the key ``0``, and the closure has ``1``.
 
+Or you can use the following parameters:
+
+.. literalinclude:: validation/041.php
+
 Available Rules
 ***************
 

user_guide_src/source/libraries/validation/041.php

@@ -0,0 +1,18 @@
+<?php
+
+$validation->setRules(
+    [
+        'foo' => [
+            'required',
+            static function ($value, $data, &$error, $field) {
+                if ((int) $value % 2 === 0) {
+                    return true;
+                }
+
+                $error = 'The value is not even.';
+
+                return false;
+            },
+        ],
+    ],
+);