Align with post-payload HTTP terminology

[annevk]

Per httpwg/http-core#655 it seems HTTP specifications are aligning on "response body" being inclusive of transfer encodings and content being response body with those removed. Per the renaming there it looks like "payload body length" has to be "content length" now, perhaps.

(It's probably not worth renaming the Body mixin and we can definitely not rename the body properties, despite them also revealing content and not the response body.)

[annevk]

With #1184 merged Fetch no longer mentions the word "payload".

I guess what we want to do is update the "Bodies" section with a note that indicates that what Fetch calls a body, HTTP calls content. (Or will soon: https://httpwg.org/http-core/draft-ietf-httpbis-semantics-latest.html#content.)

@mnot does that seem right?

[mnot]

Think so.