doc/go1.18: add information on new pacer.

This is based off Michael's notes.

Updates #47694

Change-Id: I6e7944f85b776e8481829a2fafd177a49557c6ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/368156


Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>