Add explanation where to place custom rules error messages

Author: dgvirtual

user_guide_src/source/libraries/validation.rst

@@ -760,9 +760,12 @@ 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 cescribed below – the
+fourth) parameter:
 
 .. literalinclude:: validation/035.php