message: docs(service-worker): add staleWhileRevalidate strategy
  There is great workaround for implementing staleWhileRevalidate strategy in service-worker by setting strategy to freshness and timeout to 0u. Documented this in service worker config where all other strategies are documented
  Fixes
  PR Close
diff: @@ -193,6 +193,21 @@ The Angular service worker can use either of two caching strategies for data res
  * `freshness` optimizes for currency of data, preferentially fetching requested data from the network. Only if the network times out, according to `timeout`, does the request fall back to the cache. This is useful for resources that change frequently; for example, account balances.
  +
  +<div class="alert is-helpful">
  +
  +You can also emulate a third strategy, [staleWhileRevalidate](https://developers.google.com/web/fundamentals/instant-and-offline/offline-cookbook/#stale-while-revalidate), which returns cached data (if available), but also fetches fresh data from the network in the background for next time.
  +To use this strategy set `strategy` to `freshness` and `timeout` to `0u` in `cacheConfig`.
  +
  +This will essentially do the following:
  +
  +1. Try to fetch from the network first.
  +2. If the network request does not complete after 0ms (i.e. immediately), fall back to the cache (ignoring cache age).
  +3. Once the network request completes, update the cache for future requests.
  +4. If the resource does not exist in the cache, wait for the network request anyway.
  +
  +</div>
  +
  ### `cacheQueryOptions`
  See [assetGroups](#assetgroups) for details.
  