doc/go1.18: warn about possible breakage of 1.18 generic code
doc/go1.18: warn about possible breakage of 1.18 generic code

The Go 1 compatibility guarantee permits us to break code if there is
a specification error or a bug. Emphasize that for generics.

Change-Id: I8379a14cdab9f63bb747e961ca12d1adecfc2eb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/388454
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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