docs(service-worker): add info about recovering from degraded Driver …

…state

Related to angular#31865

aio/content/guide/service-worker-devops.md

@@ -207,6 +207,9 @@ There are two possible degraded states:
 clean copy of the latest known version of the app. Older cached
 versions are safe to use, so existing tabs continue to run from
 cache, but new loads of the app will be served from the network.
+The service worker will try to recover from that state when a new
+version of the application is found, i.e. when a new `ngsw.json`
+is available.
 
 * `SAFE_MODE`: the service worker cannot guarantee the safety of
 using cached data. Either an unexpected error occurred or all
@@ -216,6 +219,10 @@ network, running as little service worker code as possible.
 In both cases, the parenthetical annotation provides the
 error that caused the service worker to enter the degraded state.
 
+Both states are temporary, as they are saved for the lifetime of ServiceWorker instance only.
+ServiceWorkers are stopped by the browser after a certain period of inactivity and a new instance is created as soon as the ServiceWorker needs to handle an event (such as a request from the page).
+Those new instances would start from the `NORMAL` mode.
+An Angular update of the `ngsw-worker.js` script will also create a new instance and thus reset the Driver state.
 
 #### Latest manifest hash