Update docs

docs/advanced-usage-zh.md

@@ -216,6 +216,8 @@ iptables -t nat -I POSTROUTING -s 192.168.43.0/24 -o "$netif" -m policy --dir ou
 iptables -t nat -I POSTROUTING -s 192.168.42.0/24 -o "$netif" -j SNAT --to 192.0.2.2
 ```
 
+**注：** 以上方法仅适用于服务器的默认网络接口对应多个公有 IP 的情况。如果服务器有多个网络接口，对应不同的公有 IP，则此方法无效。
+
 要检查一个已连接的 VPN 客户端的 "出站 IP"，你可以在该客户端上打开浏览器并到 [这里](https://www.ipchicken.com) 检测 IP 地址。
 
 ## 自定义 VPN 子网

docs/advanced-usage.md

@@ -216,6 +216,8 @@ iptables -t nat -I POSTROUTING -s 192.168.43.0/24 -o "$netif" -m policy --dir ou
 iptables -t nat -I POSTROUTING -s 192.168.42.0/24 -o "$netif" -j SNAT --to 192.0.2.2
 ```
 
+**Note:** The method above only applies if the VPN server's default network interface maps to multiple public IPs. This method may not work if the server has multiple network interfaces, each with a different public IP.
+
 To check the "outgoing IP" for a connected VPN client, you may open a browser on the client and [look up the IP address on Google](https://www.google.com/search?q=my+ip).
 
 ## Customize VPN subnets