Try to clarify wording to reduce zpool add incidents

Try to clarify wording to reduce zpool add incidents.
Add an attach example.

Reviewed-by: Rich Ercolani <Rincebrain@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #15179