tiflash: add upgrade doc for 4.0 (#2225)

TOC.md

@@ -315,6 +315,7 @@
     - [Maintain a TiFlash Cluster](/reference/tiflash/maintain.md)
     - [Monitor TiFlash](/reference/tiflash/monitor.md)
     - [Scale TiFlash](/reference/tiflash/scale.md)
+    - [Upgrade TiFlash Nodes](/reference/tiflash/upgrade.md)
     - [Configure TiFlash](/reference/tiflash/configuration.md)
   + TiDB Binlog
     - [Overview](/reference/tidb-binlog/overview.md)

reference/tiflash/upgrade.md

@@ -0,0 +1,27 @@
+---
+title: Upgrade TiFlash Nodes
+summary: Learn how to upgrade TiFlash nodes.
+category: reference
+---
+
+# Upgrade TiFlash Nodes
+
+Before the upgrade, make sure that the cluster is started. To upgrade TiFlash nodes, take the following steps:
+
+1. Run the upgrade command:
+
+    {{< copyable "shell-regular" >}}
+
+    ```shell
+    tiup cluster upgrade test v4.0.0-rc
+    ```
+
+2. View the cluster status:
+
+    {{< copyable "shell-regular" >}}
+
+    ```shell
+    tiup cluster display test
+    ```
+
+3. Access the monitoring platform using your browser, and view the status of the cluster.