docs: add warning to sample code

Author: kenjis

user_guide_src/source/libraries/validation/038.php

@@ -8,3 +8,5 @@
     'name' => "is_unique[supplier.name,uuid,{$uuid}]",   // is ok
     'name' => 'is_unique[supplier.name,uuid,{uuid}]',    // is ok - see "Validation Placeholders"
 ]);
+// Warning: If `$uuid` is a user input, be sure to validate the format of the value before using it.
+// Otherwise, it is vulnerable.