doc/go1.18: remove some TODOs for changes we aren't mentioning

Updates #47694

Change-Id: I8fdbded4aaf974425ee317a1cdd940a5ac88c0b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/368794


Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>