docs(service-worker): improve docs related to SwUpdate APIs (#46960)

This commit improves documentation related to recently improved or
deprecated `SwUpdate` APIs in the following ways:

- Update [check-for-update.service.ts][1] to make use of the return
  value of [SwUpdate#checkForUpdate()][2].
- Update [prompt-update.service.ts][3] to not call
  [SwUpdate#activateUpdate()][4] and just reload the page.
- Update the [SwUpdate#activateUpdate()][4] API docs to explain that it
  is only useful if you want to update a client without reloading and
  that it can easily lead to version skew.
- Update [a code-snippet][5] to no longer be [hard-coded][6].

[1]: https://github.com/angular/angular/blob/9d9d05911dbd6e2f30e4c7bced0e41fd20ec4285/aio/content/examples/service-worker-getting-started/src/app/check-for-update.service.ts#L16
[2]: https://angular.io/api/service-worker/SwUpdate#checkForUpdate
[3]: https://github.com/angular/angular/blob/9d9d05911dbd6e2f30e4c7bced0e41fd20ec4285/aio/content/examples/service-worker-getting-started/src/app/prompt-update.service.ts#L15
[4]: https://angular.io/api/service-worker/SwUpdate#activateUpdate
[5]: https://github.com/angular/angular/blob/96c6139c9ab35aa6ab2330a5a79a5906d5c2e8be/packages/service-worker/src/update.ts#L44-L54
[6]: https://angular.io/guide/docs-style-guide#hard-coded-snippets

Fixes #43665

PR Close #46960

packages/service-worker/src/update.ts

@@ -40,18 +40,11 @@ export class SwUpdate {
    *
    * @deprecated Use {@link versionUpdates} instead.
    *
-   * The of behavior `available` can be rebuild by filtering for the `VersionReadyEvent`:
-   * ```
-   * import {filter, map} from 'rxjs/operators';
-   * // ...
-   * const updatesAvailable = swUpdate.versionUpdates.pipe(
-   *   filter((evt): evt is VersionReadyEvent => evt.type === 'VERSION_READY'),
-   *   map(evt => ({
-   *     type: 'UPDATE_AVAILABLE',
-   *     current: evt.currentVersion,
-   *     available: evt.latestVersion,
-   *   })));
-   * ```
+   * The behavior of `available` can be replicated by using `versionUpdates` by filtering for the
+   * `VersionReadyEvent`:
+   *
+   * {@example service-worker-getting-started/src/app/prompt-update.service.ts
+   * region='sw-replicate-available'}
    */
   readonly available: Observable<UpdateAvailableEvent>;
 
@@ -124,6 +117,20 @@ export class SwUpdate {
    * Updates the current client (i.e. browser tab) to the latest version that is ready for
    * activation.
    *
+   * In most cases, you should not use this method and instead should update a client by reloading
+   * the page.
+   *
+   * <div class="alert is-important">
+   *
+   * Updating a client without reloading can easily result in a broken application due to a version
+   * mismatch between the [application shell](guide/glossary#app-shell) and other page resources,
+   * such as [lazy-loaded chunks](guide/glossary#lazy-loading), whose filenames may change between
+   * versions.
+   *
+   * Only use this method, if you are certain it is safe for your specific use case.
+   *
+   * </div>
+   *
    * @returns a promise that
    *  - resolves to `true` if an update was activated successfully
    *  - resolves to `false` if no update was available (for example, the client was already on the