docs: add link to list of validation rules

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -877,6 +877,8 @@ Or you can use the following parameters:
 .. literalinclude:: validation/047.php
    :lines: 2-
 
+.. _validation-available-rules:
+
 ***************
 Available Rules
 ***************

user_guide_src/source/models/model.rst

@@ -676,8 +676,12 @@ prior to saving to the database with the ``insert()``, ``update()``, or ``save()
 Setting Validation Rules
 ------------------------
 
-The first step is to fill out the `$validationRules`_ class property with the fields and rules that should
-be applied. If you have custom error message that you want to use, place them in the `$validationMessages`_ array:
+The first step is to fill out the `$validationRules`_ class property with the
+fields and rules that should be applied.
+
+.. note:: You can see the list of built-in Validation rules in :ref:`validation-available-rules`.
+
+If you have custom error message that you want to use, place them in the `$validationMessages`_ array:
 
 .. literalinclude:: model/027.php