Docs for the new directive in https://github.com/caddyserver/caddy/pull/4034.
Also adding a bit in `handle_errors` mentioning that `reverse_proxy` doesn't trigger errors when a response has an "error" HTTP status; not sure how to word this clearly, cause `reverse_proxy` still can trigger errors if there's no upstream or whatever. We should probably add examples for that later, and augment this paragraph to mention `handle_response` once that's merged (https://github.com/caddyserver/caddy/pull/4021)
