doc/go1.18: mention new x/exp generic packages
doc/go1.18: mention new x/exp generic packages

Change-Id: I119cfb1a0da9af89ced78935b8fcdfdb8d9b4ab6
Reviewed-on: https://go-review.googlesource.com/c/go/+/383794
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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