docs(service-worker): add section to explain unrecoverable state (#36847)

PR Close #36847

aio/content/examples/service-worker-getting-started/src/app/handle-unrecoverable-state.service.ts

@@ -0,0 +1,17 @@
+import { Injectable } from '@angular/core';
+import { SwUpdate } from '@angular/service-worker';
+
+function notifyUser(message: string): void { }
+
+// #docregion sw-unrecoverable-state
+@Injectable()
+export class HandleUnrecoverableStateService {
+  constructor(updates: SwUpdate) {
+    updates.unrecoverable.subscribe(event => {
+      notifyUser(
+        `An error occurred that we cannot recover from:\n${event.reason}\n\n` +
+        'Please reload the page.');
+    });
+  }
+}
+// #enddocregion sw-unrecoverable-state