next up: try to use a typemap for the headers

From https://www.rfc-editor.org/rfc/rfc9110.html#section-6.3:

> 6.3. Header Fields
> Fields (Section 5) that are sent or received before the content are referred to as  "header fields" (or just "headers", colloquially).

> The "header section" of a message consists of a sequence of header field lines. Each header field might modify or extend message semantics, describe the sender, define the content, or provide additional context.

> Note: We refer to named fields specifically as a "header field" when they are only allowed to be sent in the header section.

This means that our use of "header" is currently wrong. Because "fields" can be sent in both the headers and trailers section, solely referring to them as "headers" is wrong. We can work around confusion by linking to "headers" using a doc alias.