Fixed #19721 -- Allowed admin filters to customize the list separator.

docs/releases/4.1.txt

@@ -54,6 +54,10 @@ Minor features
 * The admin :ref:`dark mode CSS variables <admin-theming>` are now applied in a
   separate stylesheet and template block.
 
+* :ref:`modeladmin-list-filters` providing custom ``FieldListFilter``
+  subclasses can now control the query string value separator when filtering
+  for multiple values using the ``__in`` lookup.
+
 :mod:`django.contrib.admindocs`
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