Sync docstring in async_client

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