(DOCSP-13518) X509 username field is optional (#316)

* (DOCSP-13518) X509 username field is optional

Author: zach-carr

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

@@ -222,12 +222,17 @@ content: |
 
                  Select :guilabel:`X.509` if the deployment uses
                  :manual:`X.509 </core/security-x.509/>` as its
-                 authentication mechanism. If selected, you must
-                 provide the  :guilabel:`Username` to authenticate
-                 the user.
+                 authentication mechanism. 
 
+                 If selected, you may optionally provide the 
+                 :guilabel:`Username` to authenticate the user.
+                 
                  .. note::
 
+                    |compass-short| retrieves the username
+                    from the X.509 certificate. In most most cases, you 
+                    do not need to provide the :guilabel:`Username`.
+
                     If you are using 
                     :atlas:`Atlas-managed certificates </security-add-mongodb-users/#select-an-authentication-method>`
                     , your username must be prefaced by "CN="