Caddy doesn’t document specifics for individual HTTP headers; it doesn’t make sense to do so, that’s not really a concern of the webserver specifically. Some headers do have special meaning, and we do call those out (e.g. Host and some proxy headers like X-Forwarded-*) but most only have a meaning to clients.
We document how to match against and manipulate headers, it’s the user’s job to connect the dots to do what they need.