(DOCSP-15423): Added a note about a known issue.

Author: corryroot

source/reference/ops-manager/security-enable.txt

@@ -17,6 +17,13 @@ mechanism </tutorial/enable-mongodbcr-authentication-for-group/>`
 for the specified |mms| clusters in the given project. You can also
 enable authentication through the |mms| UI.
 
+.. note::
+
+   After you enable an authentication mechanism with {+mcli+}, the 
+   :guilabel:`Security` page of the |mms| UI shows no users for 
+   :guilabel:`Backup Auth` and :guilabel:`Montoring Auth` because these 
+   users are deprecated for new projects.
+
 Syntax
 ------