AD Review of Signatures (Editorial)

[jricher]

Collects editorial comments of AD-reviewed signatures draft.

Closes #2371
Closes #2364
Closes #2368
Closes #2333
Closes #2330

[richanna]

Changes look good to me, but the editor's copy build failed.

[jricher]

The failure seems to be in the rfc-http-validate tooling. This isn't failing on any other branch, so I'm not sure what's going on here to be honest. @martinthomson @mnot am I missing something? I thought we could use the http-message tag on message fragments, but that seems to be what's breaking from the build log here. It builds fine locally.

Nevermind, it seems the change to the trailing-space examples is what broke it. I removed the http-message tag from that one and it builds fine now. There really should be a better way to indicate a trailing space in a way the document tooling doesn't destroy.