Sync docstring in async_client

Author: lidizheng

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

@@ -60,14 +60,19 @@ class {{ service.async_client_name }}:
             transport (Union[str, ~.{{ service.name }}Transport]): The
                 transport to use. If set to None, a transport is chosen
                 automatically.
-            client_options (ClientOptions): Custom options for the client.
+            client_options (ClientOptions): Custom options for the client. It
+                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.
-                (2) If ``transport`` argument is None, ``client_options`` can be
-                used to create a mutual TLS transport. If ``client_cert_source``
-                is provided, mutual TLS transport will be created with the given
-                ``api_endpoint`` or the default mTLS endpoint, and the client
-                SSL credentials obtained from ``client_cert_source``.
+                default endpoint provided by the client. GOOGLE_API_USE_MTLS
+                environment variable can also be used to override the endpoint:
+                "always" (always use the default mTLS endpoint), "never" (always
+                use the default regular endpoint, this is the default value for
+                the environment variable) and "auto" (auto switch to the default
+                mTLS endpoint if client SSL credentials is present). However,
+                the ``api_endpoint`` property takes precedence if provided.
+                (2) The ``client_cert_source`` property is used to provide client
+                SSL credentials for mutual TLS transport. If not provided, the
+                default SSL credentials will be used if present.
 
         Raises:
             google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport