Update documentation

docs/configuration/experimental/v2ray-api.md

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+### Structure
+
+!!! quote ""
+
+    V2Ray API is not included by default, see [Installation](./#installation).
+
+```json
+{
+  "listen": "127.0.0.1:8080",
+  "stats": {
+    "enabled": true,
+    "inbounds": [
+      "socks-in"
+    ],
+    "outbounds": [
+      "proxy",
+      "direct"
+    ],
+    "users": [
+      "sekai"
+    ]
+  }
+}
+```
+
+### Fields
+
+#### listen
+
+gRPC API listening address. V2Ray API will be disabled if empty.
+
+#### stats
+
+Traffic statistics service settings.
+
+#### stats.enabled
+
+Enable statistics service.
+
+#### stats.inbounds
+
+Inbound list to count traffic.
+
+#### stats.outbounds
+
+Outbound list to count traffic.
+
+#### stats.users
+
+User list to count traffic.