Add IdentityProvider.close() to the explainer (w3c-fedid#486)

* Add IdentityProvider.close() to the explainer

* address comments

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

@@ -53,17 +53,28 @@ was the last/only account getting signed out.
 
 ### JS API
 
-And IdP can alternatively call the IdP Sign-in Status API via JS calls through the static functions `IdentityProvider.login()` and `IdentityProvider.logout()`.
+An IdP can alternatively call the IdP Sign-in Status API via JS calls through
+the static functions `IdentityProvider.login()` and
+`IdentityProvider.logout()`. These are to be called from the IDP's origin, and
+marks the current origin as signed in or signed out.
 
 ```
 [Exposed=Window]
 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