docs: add sample code of rules as array

Author: kenjis

user_guide_src/source/libraries/validation/006.php

@@ -4,3 +4,8 @@
     'username' => 'required',
     'password' => 'required|min_length[10]',
 ]);
+// or
+$validation->setRules([
+    'username' => 'required',
+    'password' => ['required', 'min_length[10]'],
+]);

user_guide_src/source/libraries/validation/007.php

@@ -4,3 +4,8 @@
     'username' => ['label' => 'Username', 'rules' => 'required'],
     'password' => ['label' => 'Password', 'rules' => 'required|min_length[10]'],
 ]);
+// or
+$validation->setRules([
+    'username' => ['label' => 'Username', 'rules' => 'required'],
+    'password' => ['label' => 'Password', 'rules' => ['required', 'min_length[10]']],
+]);