Update the docstring for client_options

Author: lidizheng

gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

@@ -150,7 +150,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
                 transport to use. If set to None, a transport is chosen
                 automatically.
             client_options (ClientOptions): Custom options for the client. It
-                won't take effect unless ``transport`` is None.
+                won't take effect if a ``transport`` instance is provided.
                 (1) The ``api_endpoint`` property can be used to override the
                 default endpoint provided by the client. GOOGLE_API_USE_MTLS
                 environment variable can also be used to override the endpoint: