Mentioned non_field_errors in form validation docs.

docs/ref/forms/validation.txt

@@ -365,7 +365,9 @@ example::
                     )
 
 In this code, if the validation error is raised, the form will display an
-error message at the top of the form (normally) describing the problem.
+error message at the top of the form (normally) describing the problem. Such
+errors are non-field errors, which are displayed in the template with
+``{{ form.non_field_errors }}``.
 
 The call to ``super().clean()`` in the example code ensures that any validation
 logic in parent classes is maintained. If your form inherits another that