x/build/cmd/relnote: missed API changes

[rsc]

Somehow we missed the addition of binary.NativeEndian, which is listed in the API files, when preparing the Go 1.21 documentation. The existence in the API files marked with #57237 should have provoked relnote to insist on a reference to go.dev/issue/57237 in go1.21.html, but there is no such mention. #62349 tracks updating the docs. This issue is about fixing the process that led to the incomplete docs.

Just before the release we noticed that we'd forgotten to document the new runtime.PanicNilError, and again it seems like relnote should have noticed and caught that for us.

Marking a release blocker for Go 1.22 because we need to understand why we are dropping release notes on the floor. Either we are not using relnote often enough or correctly or there is a bug in it.

[ianlancetaylor]

Looking at cmd/relnote, I think it's this line:

				if issue := gh.Issue(num); issue != nil && !issue.ClosedAt.Before(cutoff) && hasLabel(issue, "Proposal-Accepted") {

Note `ClosedAt. The issue in question, #57237 was not closed. I think that it wasn't closed because there were two parts to it, one for encoding/binary and one for x/sys/cpu. The two independent CLs referenced it but didn't close it. I just closed it today as I think the work is complete.