docs: add failed notice to in-page login

Author: embbnux

docs/sso-login-mode.md

@@ -1,5 +1,7 @@
 # In-page Login (Beta)
 
+**Notice:** This feature is failed.
+
 RingCentral Embeddable is a whole front-end application, it doesn't need backend server and it saves user data on local browser. But for some developers want to integrate RingCentral service in backend and front-end, RingCentral Embeddable supports to reuse login status from server-side oauth login.
 
 ## Prerequisites