[1.7.x] Fixed #24281 -- Improved docs for timezone handling for auto_now and auto_now_add

Thanks djbug for the report and Aymeric Augustin and Carl Meyer for the
review.

Backport of 8119876d4a from master

docs/ref/models/fields.txt

@@ -501,6 +501,15 @@ message key.
     ``True`` will cause the field to have ``editable=False`` and ``blank=True``
     set.
 
+.. note::
+    The ``auto_now`` and ``auto_now_add`` options will always use the date in
+    the :ref:`default timezone <default-current-time-zone>` at the moment of
+    creation or update. If you need something different, you may want to
+    consider simply using your own callable default or overriding ``save()``
+    instead of using ``auto_now`` or ``auto_now_add``; or using a
+    ``DateTimeField`` instead of a ``DateField`` and deciding how to handle the
+    conversion from datetime to date at display time.
+
 ``DateTimeField``
 -----------------