Doc'd when pre_save() is called with Model.save()'s update_fields.
@@ -621,6 +621,14 @@ from the DB will get updated. In effect there is an automatic
``update_fields`` in this case. If you assign or change any deferred field
value, the field will be added to the updated fields.
+.. note::
+
+ If ``update_fields`` is passed in, only the
+ :meth:`~django.db.models.Field.pre_save` methods of the ``update_fields``
+ are called. For example, this means that date/time fields with
+ ``auto_now=True`` will not be updated unless they are included in the
+ ``update_fields``.
+
Deleting objects
================
