(DOCS-13529) Clarify Compass X509 connections (#238)

* Add notes to X509 subtab of connect procedure

* Remove SRV note

Author: zach-carr

source/includes/steps-starting-compass-individual-fields.yaml

@@ -185,6 +185,14 @@ content: |
                  provide the  :guilabel:`Username` to authenticate
                  the user.
 
+                 .. note::
+
+                    If you are using 
+                    :atlas:`Atlas-managed certificates </security-add-mongodb-users/#select-an-authentication-method>`
+                    , your username must be prefaced by "CN="
+                    per :rfc:`2253`. For example, the username 
+                    "X509User" must be provided as "CN=X509User".
+
      * - :guilabel:`Favorite Name`
        - *Optional*. A name for the connection. To save the current
          connection entered as a