Mention brand specific last digits expectations

Author: ugexe

HISTORY.rst

@@ -28,10 +28,16 @@ History
   ``last_digits``/``last_4_digits`` also now supports two digit values in
   addition to the previous four digit values.
 * Eight digit ``/credit_card/issuer_id_number`` inputs are now supported in
-  addition to the previously accepted six digit ``issuer_id_number``. If you
-  send six digits for the ``issuer_id_number``, you should send the last four
-  digits for ``last_digits``. If you send eight digits for ``issuer_id_number``
-  you should send the last two digits for ``last_digits``.
+  addition to the previously accepted six digit ``issuer_id_number``. In most
+  cases, you should send the last four digits for ``last_digits``. If you send
+  an ``issuer_id_number`` that contains an eight digit IIN, and if the credit
+  card brand is not one of the following, you should send the last two digits
+  for ``last_digits``:
+  * ``Discover``
+  * ``JCB``
+  * ``Mastercard``
+  * ``UnionPay``
+  * ``Visa``
 
 2.5.0 (2021-09-20)
 ++++++++++++++++++