Document mutable headers on Response.json()

[deerred643-star]

Description

Document that the headers on a response created by Response.json() are mutable, contrast them with the immutable headers on a fetched response, and demonstrate changing a header.

Motivation

Readers currently have no indication that they can modify the headers after creating a JSON response.

Additional details

The Fetch Standard creates the response with a response headers guard. The proposed change for Response.redirect() is still pending in whatwg/fetch#1944, so this PR intentionally documents only the behavior that is already specified.

Related issues and pull requests

Relates to mdn/mdn#858.

[wbamberg]

I think this PR over-emphasises the mutable thing. Header mutability is already documented in https://developer.mozilla.org/en-US/docs/Web/API/Headers#modification_restrictions, so I don't really think it is needed here, and if it were needed, it should be done consistently across all the places that can give you a Response.

But even then, this PR makes it seem as if header mutability is the main point of the API, by for instance describing the return value as "A {{domxref("Response")}} object whose {{domxref("Response.headers", "headers")}} are mutable.". But that's really a detail about the API AFAICS.