Adding more concrete guidance about digest processing and validation

Hi folks,

During our IESG review of the Digest fields draft, Roman Danyliw raised
some comments related to the expectations on receivers to validate digests.
Roman has proposed some additional language for concrete guidance and I
think this is good. Its effectively a restatement of something that was
implicit, using normative language. As such I don't think this is a major
change but I wanted to raise the visibility in the group in case anyone had
any opinions.

Please comment on the issue
https://github.com/httpwg/http-extensions/issues/2557 or PR
https://github.com/httpwg/http-extensions/pull/2558

Cheers

Lucas