docs: Add documentation for config setting CMS_ADMIN_NAMESPACE. Resolves

 #4105. (#7262)

Co-authored-by: crydotsnake <simondotunix@gmail.com>
Co-authored-by: Simon Krull <krull@punkt.de>

docs/reference/configuration.rst

@@ -408,8 +408,8 @@ A list of plugin processors. Plugin processors are callables that modify all
 plugins' output *after* rendering. See :doc:`/how_to/custom_plugins`
 for more information.
 
-..  setting:: CMS_APPHOOKS
 
+..  setting:: CMS_APPHOOKS
 
 CMS_APPHOOKS
 ============
@@ -433,6 +433,22 @@ Example::
     )
 
 
+..  setting:: CMS_ADMIN_NAMESPACE
+
+CMS_ADMIN_NAMESPACE
+=====================
+
+default
+    ``admin``
+
+Used to configure the namespace of the admin interface.
+
+When using a custom instance of Django's
+:class:`~django.contrib.admin.AdminSite`, provide the unique instance name as
+``CMS_ADMIN_NAMESPACE``. This instance name is used when :ref:`reversing admin
+URLs <django:admin-reverse-urls>`.
+
+
 .. _i18n_l10n_reference:
 
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