Clarified "AbstractUser.last_login allows null values" in 1.8 release notes.

The :attr:`AbstractUser.last_login <django.contrib.auth.models.User.last_login>`

field now allows null values. Previously, it defaulted to the time when the user

was created which was misleading if the user never logged in. Please run the

database migration. If your custom user inherits from ``AbstractUser`` and you

wish to set ``last_login`` to ``NULL`` for users who haven't logged in, you can

run this query::

was created which was misleading if the user never logged in. If you are using

the default user (:class:`django.contrib.auth.models.User`), run the database

migration included in ``contrib.auth``.

If you are using a custom user model that inherits from ``AbstractUser``,

you'll need to run :djadmin:`makemigrations` and generate a migration for your

app that contains that model. Also, if wish to set ``last_login`` to ``NULL``

for users who haven't logged in, you can run this query::

    from django.db import models

    from django.contrib.auth import get_user_model