Add statsUserOnline Usage

---------

Co-authored-by: 风扇滑翔翼 <Fangliding.fshxy@outlook.com>

docs/en/config/policy.md

@@ -17,6 +17,7 @@ Local policy can be used to set different policy settings for different user lev
         "downlinkOnly": 5,
         "statsUserUplink": false,
         "statsUserDownlink": false,
+        "statsUserOnline": false,
         "bufferSize": 4
       }
     },
@@ -84,6 +85,10 @@ When set to `true`, enables upstream traffic statistics for all users at the cur
 
 When set to `true`, enables downstream traffic statistics for all users at the current level.
 
+> `statsUserOnline`: true | false
+
+When set to `true`, enables counting online users for all users at the current level by email.
+
 > `bufferSize`: number
 
 The internal buffer size of each connection. Unit: kB. When set to `0`, the internal buffer is disabled.