Fixed #17707, #20734 -- Added examples to URL namespacing docs.

Thanks Daniele Procida for review.

docs/ref/contrib/admin/index.txt

@@ -2519,6 +2519,7 @@ own ``AdminSite`` instance since you will likely be importing all the per-app
 put ``'django.contrib.admin.apps.SimpleAdminConfig'`` instead of
 ``'django.contrib.admin'`` in your :setting:`INSTALLED_APPS` setting.
 
+.. _multiple-admin-sites:
 
 Multiple admin sites in the same URLconf
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