Add IdentityProvider.close() to the explainer

proposals/idp-sign-in-status-api.md

@@ -60,11 +60,20 @@ And IdP can alternatively call the IdP Sign-in Status API via JS calls through t
 interface IdentityProvider {
   static void login();
   static void logout();
+
+  static void close();
 }
 ```
 
 To be called from the IDP's origin, and marks the current origin as signed in or signed out.
 
+In addition, a `close()` function is provided to signal to the browser that the
+signin flow is finished. The reason for this function in addition to the header
+is that even when the user is already logged in, the signin flow may not be
+finished yet; for example, an IDP may want to prompt the user to verify their
+phone number. To allow for such flows, the IDP must call
+`IdentityProvider.close()` when the flow is fully done.
+
 ### Config file addition
 
 This proposal adds a new `signin_url` field to [the config file](https://fedidcg.github.io/FedCM/#dictdef-identityproviderapiconfig):