TUN inbound: Add desc config for Windows ("Wintun" by default) (#6486)

https://github.com/XTLS/Xray-docs-next/pull/872#issuecomment-4936055488

proxy/tun/README.md

@@ -31,13 +31,17 @@ Here is simple Xray config snippet to enable the inbound:
       "port": 0,
       "protocol": "tun",
       "settings": {
-        "name": "xray0",
-        "MTU": 1492
+        "name": "utun10",
+        "desc": "Wintun",
+        "mtu": 1500
       }
     }
   ],
 ```
 
+`desc` sets the Windows Wintun adapter tunnel type and defaults to `Wintun`.
+It is ignored on other platforms.
+
 ## SUPPORTED FEATURES
 
 - IPv4 and IPv6