doc/go1.18: add information on new pacer.
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 <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
Run-TryBot: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

CAFxX/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go