message: Docs: add_user_agent option needs to be set for x-envoy-downstream-cluster scenarios
diff: @@ -48,7 +48,8 @@ external requests, but for internal requests will contain the service cluster of
  that in the current implementation, this should be considered a hint as it is set by the caller and
  could be easily spoofed by any internal entity. In the future Envoy will support a mutual
  authentication TLS mesh which will make this header fully secure. Like *user-agent*, the value
  -is determined by the :option:`--service-cluster` command line option.
  +is determined by the :option:`--service-cluster` command line option. In order to enable this
  +feature you need to set the :ref:`user_agent <config_http_conn_man_add_user_agent>` option to true.
  .. _config_http_conn_man_headers_x-envoy-external-address:
  