Add note to log out from IS

src/IdentityAuthClient.js

@@ -69,6 +69,7 @@ export default class IdentityAuthClient {
         // appropriately. We already clear storage on sign out, but we'll need
         // additional clearing when changing ISes in settings as part of future
         // privacy work.
+        // See also https://github.com/vector-im/riot-web/issues/10455.
     }
 
     async registerForToken() {