docs: expand doc for app_initializer token (#34760)

PR Close #34760

packages/core/src/application_init.ts

@@ -12,7 +12,15 @@ import {Inject, Injectable, InjectionToken, Optional} from './di';
 
 
 /**
- * A function that will be executed when an application is initialized.
+ * An injection token that allows you to provide one or more initialization functions.
+ * These function are injected at application startup and executed during
+ * app initialization. If any of these functions returns a Promise, initialization
+ * does not complete until the Promise is resolved.
+ *
+ * You can, for example, create a factory function that loads language data
+ * or an external configuration, and provide that function to the `APP_INITIALIZER` token.
+ * That way, the function is executed during the application bootstrap process,
+ * and the needed data is available on startup.
  *
  * @publicApi
  */