doc/go1.18: add docs on User.GroupIds

doc/go1.18: add docs on User.GroupIds

Cobbled up some release notes from:

https://go-review.googlesource.com/c/go/+/330753/

Updates #47694

Change-Id: I249e4b96b7d51185b07c11a734505677e9e36315
Reviewed-on: https://go-review.googlesource.com/c/go/+/368358
Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>