docs: clarify url vs code fields in qrcode event

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/channel/weixin.js

@@ -312,6 +312,8 @@ async function _runLogin({ callId, timeoutMs, botType, emit, isAborted }) {
     emit({
       action: 'event',
       event:  'qrcode',
+      // url  = qrcode_img_content: the WeChat URL encoded inside the QR image (use this for QR rendering)
+      // code = qrcode: the polling ticket used by _pollQRStatus (NOT for QR rendering)
       data:   { url: activeLogin.qrcodeUrl, code: activeLogin.qrcode, expire: 30, index: qrRefreshCount },
     });
     log.info('weixin', `callId=${callId} qrcode emitted index=${qrRefreshCount}`);