doc/go1.18: add caution about use of generics in production
doc/go1.18: add caution about use of generics in production

Per https://groups.google.com/g/golang-dev/c/iuB22_G9Kbo/m/7B1jd1I3BQAJ.

For #47694

Change-Id: I033cdadb2067e432f7c307d1546b4c5d0cfd5d8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/371954
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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