Update aliyun-sms.js

Add comments for the changed code.

server/notification-providers/aliyun-sms.js

@@ -92,6 +92,10 @@ class AliyunSMS extends NotificationProvider {
             let key = oa[i];
             param2[key] = param[key];
         }
+        
+        // Escape more characters than encodeURIComponent does.
+        // For generating Aliyun signature, all characters except A-Za-z0-9~-._ are encoded.
+        // See https://help.aliyun.com/document_detail/315526.html
 
         let moreEscapesTable = function (m) {
             return {