Add more description of vpc-nat-gateway

(cherry picked from commit e44d814)

Author: oilbeater

Diff for: docs/guide/vpc.en.md

@@ -137,7 +137,7 @@ spec:
     }'
 ```
 
-- This Subnet is used to manage the available external addresses, so please communicate with your network management to give you the available physical segment IPs.
+- This Subnet is used to manage the available external addresses and the address will be allocated to VPC NAT Gateway through Macvlan, so please communicate with your network management to give you the available physical segment IPs.
 - The VPC gateway uses Macvlan for physical network configuration, and `master` of `NetworkAttachmentDefinition` should be the NIC name of the corresponding physical network NIC.
 - `provider` format is `<NetworkAttachmentDefinition Name>.<NetworkAttachmentDefinition Namespace>`.
 - `name` must be `ovn-vpc-external-network`.

Diff for: docs/guide/vpc.md

@@ -134,7 +134,7 @@ spec:
     }'
 ```
 
-- 该 Subnet 用来管理可用的外部地址，请和网络管理沟通给出可用的物理段 IP。
+- 该 Subnet 用来管理可用的外部地址，网段内的地址将会通过 Macvlan 分配给 VPC 网关，请和网络管理沟通给出可用的物理段 IP。
 - VPC 网关使用 Macvlan 做物理网络配置，`NetworkAttachmentDefinition` 的 `master` 需为对应物理网路网卡的网卡名。
 - `provider` 格式为 `<NetworkAttachmentDefinition Name>.<NetworkAttachmentDefinition Namespace>`。
 - `name` 必须为 ovn-vpc-external-network，这里代码中做了硬编码。