[1.8.x] Updated location of database backend data_types attribute in docs.

If :meth:`.get_internal_type` returns a string that is not known to Django for

the database backend you are using -- that is, it doesn't appear in

``django.db.backends.<db_name>.creation.data_types`` -- the string will still be

used by the serializer, but the default :meth:`~Field.db_type` method will

return ``None``. See the documentation of :meth:`~Field.db_type` for reasons why

this might be useful. Putting a descriptive string in as the type of the field

for the serializer is a useful idea if you're ever going to be using the

serializer output in some other place, outside of Django.

``django.db.backends.<db_name>.base.DatabaseWrapper.data_types`` -- the string

will still be used by the serializer, but the default :meth:`~Field.db_type`

method will return ``None``. See the documentation of :meth:`~Field.db_type`

for reasons why this might be useful. Putting a descriptive string in as the

type of the field for the serializer is a useful idea if you're ever going to

be using the serializer output in some other place, outside of Django.

.. _converting-model-field-to-serialization: