[1.8.x] Fixed #24690 -- Added a warning about mutable defaults for ArrayField.

    <ArrayField.size>`. ``ArrayField`` can be nested to store multi-dimensional

    arrays.

    If you give the field a :attr:`~django.db.models.Field.default`, ensure

    it's a callable such as ``list`` (for an empty default) or a callable that

    returns a list (such as a function). Incorrectly using ``default=[]``

    creates a mutable default that is shared between all instances of

    ``ArrayField``.

    .. attribute:: base_field

        This is a required argument.