Fixed #23473 -- Documented that @deconstructible classes need __eq__.

Django will write out the value as an instantiation of your class with the

given arguments, similar to the way it writes out references to Django fields.

To prevent a new migration from being created each time

:djadmin:`makemigrations` is run, you should also add a ``__eq__()`` method to

the decorated class. This function will be called by Django's migration

framework to detect changes between states.

As long as all of the arguments to your class' constructor are themselves

serializable, you can just use the ``@deconstructible`` class decorator

from ``django.utils.deconstruct`` to add the method::

serializable, you can use the ``@deconstructible`` class decorator from

``django.utils.deconstruct`` to add the ``deconstruct()`` method::

    from django.utils.deconstruct import deconstructible

    class MyCustomClass(object):

        def __init__(self, foo=1):

            self.foo = foo

            ...

        def __eq__(self, other):

            return self.foo == other.foo

The decorator adds logic to capture and preserve the arguments on their

way into your constructor, and then returns those arguments exactly when

deconstruct() is called.