dns_challenge_override_domain: clarify expected domain and DNS plugin support

[rom4nik]

For context: I have a CNAME record that makes _acme-challenge.domain1.com point to _acme-challenge.domain2.com.

1. When I was searching online for how to use DNS-01 challenge delegation with Caddy, I was constantly running into pages about DuckDNS plugin's support for this feature (e.g. https://caddy.community/t/can-i-use-dns-alias-mode-to-issue-a-cert-with-caddy/11877). For some reason the dns_challenge_override_domain config option (added in Support for DNS challenge delegation caddy#4071) didn't really pop up in my searches until today, and even then I thought I would have to patch the OVH plugin, since it wasn't logging any errors with the override domain set to domain2.com. It would just timeout and throw an error: no memory of presenting a DNS record for "domain2.com" (usually OK if presenting also failed).

2. This post: https://caddy.community/t/global-dns-challenge-and-dns-challenge-override-domain/16773/3 was written for DuckDNS plugin's own override_domain option, and as such it said that _acme-challenge mustn't be prepended to that option's value in Caddyfile. I initially assumed this would be true for OVH plugin as well, despite not having custom override code in it.
   On the other hand, this post: https://caddy.community/t/issuing-a-cert-with-the-dns-challenge-override-domain-directive-is-not-working-while-possible-to-issue-a-cert-for-the-alias-domain-itself/18477/2 made it clear to me that the domain set for Caddy's own dns_challenge_override_domain will be used as-is with (any?) DNS plugin.

That was pretty confusing, so I think it would be good to have it clarified in official docs.

[mholt]

I suppose it's not a bad idea to clarify these things. Thanks!

[francislavoie]

Sorry, I saw this a few days ago but forgot to follow up.

👍 from me!