Merge pull request #8343 from dgvirtual/custom_rule_error_message_placement

docs: [Validation] Add explanation where to place custom validation rules error messages

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -762,9 +762,12 @@ a boolean true or false value signifying true if it passed the test or false if
 
 .. literalinclude:: validation/034.php
 
-By default, the system will look within **system/Language/en/Validation.php** for the language strings used
-within errors. In custom rules, you may provide error messages by accepting a ``&$error`` variable by reference in the
-second parameter:
+By default, the system will look within **system/Language/en/Validation.php** for the language strings used within
+errors. To provide default error messages for your custom rules, you may place them in **app/Language/en/Validation.php**
+(and/or corresponding folder of locale you use in place of ``en``). Also, in case you want to use some other language
+string file in place of the default **Validation.php**, you may provide error messages by accepting an ``&$error``
+variable by reference in the second (or, in case your rule needs to work with parameters, as described below – the
+fourth) parameter:
 
 .. literalinclude:: validation/035.php