More terse langauge + link to server docs

Author: johnnyshields

docs/reference/configuration.txt

@@ -168,11 +168,10 @@ can be configured.
           # on 2.4 and 2.6 is :plain)
           auth_mech: :scram
 
-          # Specify the auth source, i.e. the database or other source
-          # which contains the user's login credentials.
-          # Please review the server documentation for the authentication
-          # mechanism you are using for more information on auth source(s)
-          # that can be used with that authentication mechanism.
+          # Specify the auth source, i.e. the database or other source which
+          # contains the user's login credentials. Allowed values for auth source
+          # depend on the authentication mechanism, as explained the server documentation:
+          # https://docs.mongodb.com/manual/reference/connection-string/#mongodb-urioption-urioption.authSource
           # If no auth source is specified, the default auth source as
           # determined by the driver will be used. Please refer to:
           # https://docs.mongodb.com/ruby-driver/current/reference/authentication/#auth-source