docs: move &$error description to numbered list

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -695,6 +695,7 @@ If your method needs to work with parameters, the function will need a minimum o
 1. the value to validate (``$value``)
 2. the parameter string (``$params``)
 3. an array with all of the data that was submitted the form (``$data``)
+4. (optional) a custom error string (``&$error``), just as described above.
 
 .. warning:: The other field values in ``$data`` is unvalidated (or maybe invalid)
     data. Using unvalidated input data is a source of vulnerability. You must
@@ -706,8 +707,6 @@ for rules like ``required_with`` that needs to check the value of another submit
 
 .. literalinclude:: validation/037.php
 
-Custom errors can be returned as the fourth parameter ``&$error``, just as described above.
-
 .. _validation-using-closure-rule:
 
 Using Closure Rule