Resumable Upload: Clarifications around Upload-Limit

[Acconut]

This PR clarifies a few things around the Upload-Limit field from #2747:

• The last upload append request may now be smaller than the minimum append size specified in min-append-size. For example, if a 510 MB upload is split into 100 MB requests, the last request can just be 10 MB.
• It clarified how a client can indicate the entire upload length when creating a new upload via the Content-Length header. The PR also mentions that no resumable upload resource may be created if the minimum size is not met, but the server might still accept and process the data (although without offering resumability)
• The Upload-Limit header is added to informational response in an example