add ref

Author: lindseymoore

source/connect/mongoclient.txt

@@ -59,6 +59,8 @@ A standard connection string includes the following components:
 
      - Optional. Authentication credentials. If you include these, the client
        authenticates the user against the database specified in ``authSource``.
+       For more information about the ``authSource`` connection option, 
+       see :ref:`kotlin-sync-auth`. 
 
    * - ``host[:port]``