- Add doc to syscall.Errno (and syscall.ErrorString for plan9).

- Mention under `syscall` in release notes.

Fixes #33436.

Change-Id: I032ffebaa76ed67eb9d748e7645ca73f26144ea0
Reviewed-on: https://go-review.googlesource.com/c/go/+/191337Reviewed-by: Ian Lance Taylor <iant@golang.org>