[1.10.x] Fixed #26807 -- Documented how to replicate SubfieldBase's assignment behavior.

Backport of 518eaf1fa2 from master

docs/releases/1.8.txt

@@ -1477,9 +1477,13 @@ It's a legacy option that should no longer be necessary.
 will be removed in Django 1.10. Historically, it was used to handle fields where
 type conversion was needed when loading from the database, but it was not used
 in ``.values()`` calls or in aggregates. It has been replaced with
-:meth:`~django.db.models.Field.from_db_value`. Note that the new approach does
-not call the :meth:`~django.db.models.Field.to_python` method on assignment
-as was the case with ``SubfieldBase``.
+:meth:`~django.db.models.Field.from_db_value`.
+
+The new approach doesn't call the :meth:`~django.db.models.Field.to_python`
+method on assignment as was the case with ``SubfieldBase``. If you need that
+behavior, reimplement the ``Creator`` class `from Django's source code
+<https://github.com/django/django/blob/stable/1.8.x/django/db/models/fields/subclassing.py#L31-L44>`_
+in your project.
 
 ``django.utils.checksums``
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