Fixed #19402 - Clarified purpose of CustomUser.REQUIRED_FIELDS

    .. attribute:: REQUIRED_FIELDS

        A list of the field names that *must* be provided when creating

        a user. For example, here is the partial definition for a User model

        that defines two required fields - a date of birth and height::

        A list of the field names that *must* be provided when creating a user

        via the :djadmin:`createsuperuser` management command. The user will be

        prompted to supply a value for each of these fields. It should include

        any field for which :attr:`~django.db.models.Field.blank` is ``False``

        or undefined, but may include additional fields you want prompted for

        when a user is created interactively. However, it will not work for

        :class:`~django.db.models.ForeignKey` fields.

        For example, here is the partial definition for a ``User`` model that

        defines two required fields - a date of birth and height::

            class MyUser(AbstractBaseUser):

                ...