OutputPage: Mark MakeGlobalVariablesScriptHook as void and improve docs

No extension is meant to be able to prevent other extensions from
using this hook. Incidents of that have only been accidents.

Change-Id: I048e3ac4a6a6889fc55681b8d49d6fd433e8b3fc

includes/Hook/MakeGlobalVariablesScriptHook.php

@@ -13,16 +13,22 @@ use OutputPage;
  */
 interface MakeGlobalVariablesScriptHook {
 	/**
-	 * This hook is called at end of OutputPage::getJSVars.
-	 * Ideally, this hook should only be used to add variables that depend on
-	 * the current page/request; static configuration should be added through
-	 * ResourceLoaderGetConfigVars instead.
+	 * Export user- or page-specific `mw.config` variables to JavaScript.
+	 *
+	 * When using this hook, be as selective as possible about when the data is set.
+	 * Reduce the cost by setting values only for specific titles, namespaces, or user-rights.
+	 *
+	 * Data exported here is transmitted with the highest possible bandwidth priority (ahead of
+	 * page content even). Any data that is not dependant on the current request, should go
+	 * through MediaWiki\ResourceLoader\Hook\ResourceLoaderGetConfigVarsHook instead.
+	 *
+	 * This hook is called from OutputPage::getJSVars.
 	 *
 	 * @since 1.35
 	 *
 	 * @param array &$vars Variable (or multiple variables)
 	 * @param OutputPage $out OutputPage which called the hook, can be used to get the real title
-	 * @return bool|void True or no return value to continue or false to abort
+	 * @return void This hook must not abort, it must return no value
 	 */
-	public function onMakeGlobalVariablesScript( &$vars, $out );
+	public function onMakeGlobalVariablesScript( &$vars, $out ) : void;
 }

includes/HookContainer/HookRunner.php

@@ -2539,10 +2539,11 @@ class HookRunner implements
 		);
 	}
 
-	public function onMakeGlobalVariablesScript( &$vars, $out ) {
-		return $this->container->run(
+	public function onMakeGlobalVariablesScript( &$vars, $out ) : void {
+		$this->container->run(
 			'MakeGlobalVariablesScript',
-			[ &$vars, $out ]
+			[ &$vars, $out ],
+			[ 'abortable' => false ]
 		);
 	}