Fix-ups to comments and User interface usages

* Use isRegistered() from the more narrow UserIdentity interface.
* More specific type hints.
* Remove comments that literally repeat the code.

Change-Id: Icd0eaf8236be41b258efcf81d581540d2448e9e3

includes/Api/ApiULSSetLanguage.php

@@ -67,7 +67,7 @@ class ApiULSSetLanguage extends ApiBase {
 		}
 
 		$user = $this->getUser();
-		if ( $user->isAnon() ) {
+		if ( !$user->isRegistered() ) {
 			if ( $this->getConfig()->get( 'ULSAnonCanChangeLanguage' ) ) {
 				// Anonymous users can change language.
 				// Use a cookie that also can changed by JavaScript.