添加 `loopback` 出站文档内容 (#477)

* Add missing `loopback` (Chinese version)

* Add `loopback` (Einglish version)

* Clearify

docs/config/outbounds/loopback.md

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+# Loopback
+
+Loopback 是个出站数据协议，其作用为将经该出站传出的数据重新送入路由入站，以达到数据无需离开 Xray-core 即可再次被路由处理的效果。
+
+## OutboundConfigurationObject
+
+```json
+{
+  "inboundTag": "TagUseAsInbound"
+}
+```
+
+> `inboundTag`: string
+
+用于重新路由的入站协议标识。
+
+该标识可以在路由中用于 `inboundTag` ，表示该出站中的数据可以被对应的路由规则再次处理。

docs/en/config/outbounds/loopback.md

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+# Loopback
+
+Loopback is an outbound protocol. It can send traffics through corresponding outbound to routing inbound, thus rerouting traffics to other routing rules without leaving Xray-core.
+
+## OutboundConfigurationObject
+
+```json
+{
+  "inboundTag": "TagUseAsInbound"
+}
+```
+
+> `inboundTag`: string
+
+Use as an inbound tag for routing.
+
+This tag can be used as `inboundTag` in routing rules, all traffics going through this outbound can be rerouted with routing rules with corresponding inbound tag.