Add comment explaining extension boot process

src/Extension/ExtensionServiceProvider.php

@@ -24,6 +24,10 @@ class ExtensionServiceProvider extends AbstractServiceProvider
         $this->app->singleton(ExtensionManager::class);
         $this->app->alias(ExtensionManager::class, 'flarum.extensions');
 
+        // Boot extensions when the app is booting. This must be done as a boot
+        // listener on the app rather than in the service provider's boot method
+        // below, so that extensions have a chance to register things on the
+        // container before the core boot code runs.
         $this->app->booting(function (Container $app) {
             $app->make('flarum.extensions')->extend($app);
         });