Fixed #22959 -- Documented that class-based validators need to be deconstructible.

    class MyForm(forms.Form):

        even_field = forms.IntegerField(validators=[validate_even])

You can also use a class with a ``__call__()`` method for more complex or

configurable validators. :class:`RegexValidator`, for example, uses this

technique. If a class-based validator is used in the

:attr:`~django.db.models.Field.validators` model field option, you should make

sure it is :ref:`serializable by the migration framework

<migration-serializing>` by adding :ref:`deconstruct()

<custom-deconstruct-method>` and ``__eq__()`` methods.

How validators are run

======================