It already implied that Cmd.Wait is more than os.Process.Wait, but say
so explicitly.

See https://github.com/golang/go/issues/18874#issuecomment-309921486

Updates #18874

Change-Id: Iaa46defd776ae0be817d9f4466a99ac78cfd672b
Reviewed-on: https://go-review.googlesource.com/47650Reviewed-by: Russ Cox <rsc@golang.org>