doc: document cmd/vet changes for generics in 1.18

Fixes #50011
Updates #47694

Change-Id: Id3d43f2f72de61b360b79c2b375ca1372d5f4692
Reviewed-on: https://go-review.googlesource.com/c/go/+/369979


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tim King <taking@google.com>