Update permit_empty rule description in validation guide

Clarify the behavior of the 'permit_empty' validation rule.

Author: wipped

user_guide_src/source/libraries/validation.rst

@@ -976,7 +976,7 @@ not_in_list             Yes        Fails if field is within a predetermined
 numeric                 No         Fails if field contains anything other than
                                    numeric characters.
 permit_empty            No         Allows the field to receive an empty array,
-                                   empty string, null or false. When true, 
+                                   empty string, null or false. On success, it
                                    skips all other validation rules except for
                                    required_with and required_without.
 regex_match             Yes        Fails if field does not match the regular     ``regex_match[/regex/]``