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 <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>