Clarify when a server MUST supply an upload-length

[LPardue]

We have the requirement "if the client knows the representation data's length,
it SHOULD include the Upload-Length". Which means the server can be a position
to either know, or not know the value.

This change makes a minor tweak to the text to highlight that when the server
was given a length it really MUST respond with it but when it didn't get given
one it can't.

Fixes #3187

[guoye-zhang]

The client can also specify the complete upload length implicitly through the combination of Content-Length and Upload-Complete: ?1. Wondering if the phrasing should be "unless the total length is unknown".

[LPardue]

or "unless the client has not supplied one (either by supplying its own upload-length or providing an Upload-Complete:?1 with a Content-Length)"

[guoye-zhang]

Yeah, that works

[Acconut]

The text could also just reference Section 4.1.3, which explains how the length of an upload can be discovered:

There are two different ways for the client to indicate and the upload resource to discover the length from requests for creating the upload resource (Section 4.2) or appending to it (Section 4.4):

• If the request includes the Upload-Complete field value set to true and a valid Content-Length header field, the request content is the remaining representation data. The length is then the sum of the current offset (Section 4.1.1) and the Content-Length header field value.
• The request can include the Upload-Length request and response header field. Upload-Length is an Item Structured Header Field ([STRUCTURED-FIELDS]). Its value is a non-negative Integer (Section 3.3.1 of [STRUCTURED-FIELDS]) and indicates the number of bytes of the entire representation data. Other values MUST cause the entire header field to be ignored.

(from https://httpwg.org/http-extensions/draft-ietf-httpbis-resumable-upload.html#section-4.1.3-2)

Maybe something along these lines:

Suggested change

	- MUST include the length in the `Upload-Length` header field unless the client has not supplied one, either by providing its own `Upload-Length` ({{upload-length}}) or providing an Upload-Complete:?1 with a Content-Length ({{upload-complete}}),
	- MUST include the length in the `Upload-Length` header field unless the client has not supplied one, by providing the corresponding headers as described in Section 4.1.3,

That might be a bit more watertight.

[LPardue]

I'm happy to avoid restating requirements in multiple places in the document but I'd probably want to land #3200 first

[LPardue]

I plan to rebase this and act on @Acconut's suggestion later today