Fixed #34419 -- Doc'd django.contrib.admin.sites.all_sites.

docs/ref/contrib/admin/index.txt

@@ -2831,6 +2831,10 @@ linked to the document in ``{% block dark-mode-vars %}``.
     See :ref:`customizing-adminsite` for an example of customizing the
     :class:`AdminSite` class.
 
+.. data:: django.contrib.admin.sites.all_sites
+
+    A :class:`~weakref.WeakSet` contains all admin site instances.
+
 ``AdminSite`` attributes
 ------------------------