tiup: add description in tiup upgrade doc (#2368) (#2393)

Author: sre-bot

how-to/upgrade/using-tiup.md

@@ -148,7 +148,11 @@ This section describes how to perform a rolling update to the TiDB cluster and h
 tiup cluster upgrade <cluster-name> v4.0.0-rc
 ```
 
-Performing the rolling update to the cluster will update all components one by one. During the upgrade of TiKV, all leaders in a TiKV instance are evicted before stopping the instance. The default timeout time is 10 minutes. The instance is directly stopped after this timeout time.
+Performing the rolling update to the cluster will update all components one by one. During the upgrade of TiKV, all leaders in a TiKV instance are evicted before stopping the instance. The default timeout time is 5 minutes. The instance is directly stopped after this timeout time.
+
+To perform the upgrade immediately without evicting the leader, specify `--force` in the command above. This method causes performance jitter but not data loss.
+
+To keep a stable performance, make sure that all leaders in a TiKV instance are evicted before stopping the instance. You can set `--transfer-timeout` to a super large value, for example, `--transfer-timeout 100000000` (unit: second).
 
 ### Verify the cluster version