DNS options for buildkitd.toml aren't documented in the docs

[mbentley]

This PR adds support for [dns] options: https://github.com/moby/buildkit/pull/1040

I don't see that documented in the buildkitd.toml: https://github.com/moby/buildkit/blob/master/docs/buildkitd.toml.md

Is there another authoritative source that one should look at for a complete set of options or was this just missed?