caddy-frontend: Cover failover URL with cache

Configuring failover URL and being able to rely on cached content is not so
simple, thus document the requirement and prove in test, that such
configuration works correctly.

software/caddy-frontend/README.caddy_frontend.rst

@@ -233,6 +233,8 @@ Please be aware that the `health-check-timeout` is really short by default, so i
 
 Thanks to using health-check it's possible to configure failover system. By providing `health-check-failover-url` or `health-check-failover-https-url` some special backend can be used to reply in case if original backend replies with error (codes like `5xx`). As a note one can setup this failover URL like `https://failover.example.com/?p=` so that the path from the incoming request will be passed as parameter. Additionally authentication to failover URL is supported with `health-check-authenticate-to-failover-backend` and SSL Proxy verification with `health-check-failover-ssl-proxy-verify` and `health-check-failover-ssl-proxy-ca-crt`.
 
+**Note**: It's important to correctly configure failover URL response, especially in case if it's expected to use `stale-if-error` simulation available while `enable_cache` is used. In order to serve pages from cache the failover URL have to return error HTTP code (like 503 SERVICE_UNAVAILABLE), so that in such case cached page will have precedence over the reply from failover URL.
+
 Examples
 ========