doc/go1.18: add changes to regexp to release notes
doc/go1.18: add changes to regexp to release notes

CL 354569

Updates #47694

Change-Id: I78536c110215b3c9f247c1420bcaa5fc3d8fb930
Reviewed-on: https://go-review.googlesource.com/c/go/+/368795
Trust: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
Run-TryBot: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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