docs(service-worker): improve SwRegistrationOptions docs and add example (#21842)

PR Close #21842

packages/examples/service-worker/registration-options/ngsw-worker.js

@@ -0,0 +1,14 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+// Mock `ngsw-worker.js` used for testing the examples.
+// Immediately takes over and unregisters itself.
+self.addEventListener('install', evt => evt.waitUntil(self.skipWaiting()));
+self.addEventListener(
+    'activate',
+    evt => evt.waitUntil(self.clients.claim().then(() => self.registration.unregister())));