op-guide: update timezone variable

Via: pingcap/docs-cn#877

op-guide/ansible-deployment.md

@@ -507,8 +507,7 @@ To enable the following control variables, use the capitalized `True`. To disabl
 | cluster_name | the name of a cluster, adjustable |
 | tidb_version | the version of TiDB, configured by default in TiDB-Ansible branches |
 | process_supervision | the supervision way of processes, systemd by default, supervise optional |
-| timezone | the timezone of the managed node, adjustable, `Asia/Shanghai` by default, used together with the `set_timezone` variable |
-| set_timezone | to edit the timezone of the managed node, True by default; False means closing |
+| timezone | the global default time zone configured when a new TiDB cluster bootstrap is initialized; you can edit it later using the global `time_zone` system variable and the session `time_zone` system variable as described in [Time Zone Support](../sql/time-zone.md); the default value is `Asia/Shanghai` and see [the list of time zones](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) for more optional values |
 | enable_firewalld | to enable the firewall, closed by default; to enable it, add the ports in [network requirements](recommendation.md#network-requirements) to the white list |
 | enable_ntpd | to monitor the NTP service of the managed node, True by default; do not close it |
 | set_hostname | to edit the hostname of the mananged node based on the IP, False by default |