add notice about scaling in pd node

scale-tidb-using-tiup.md

@@ -247,6 +247,10 @@ If you want to remove a TiKV node from the `10.0.1.5` host, take the following s
 >
 > You can take similar steps to remove the TiDB and PD node.
 
+> **Note:**
+>
+> The PD Client in TiKV caches the list of PD nodes. The current version of TiKV has a mechanism to automatically and regularly update PD nodes, which can help avoid that the list of PD nodes cached by TiKV is old. However, after expanding new PD, you should try to avoid directly shrinking all PD nodes existing before the expansion. If necessary, before making all the previously existing PD nodes offline, make sure to switch the PD leader to the newly expanded PD nodes.
+
 1. View the node ID information:
 
     {{< copyable "shell-regular" >}}