document the pass backend and default behavior
document the pass backend and default behavior

Also add a big warning about cleartext passwords.

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@docker.com>

allencloud/cli,cpuguy83/cli,dnephin/cli,thaJeztah/cli,ashfall/cli,dnephin/cli,docker/cli,ashfall/cli,allencloud/cli,thaJeztah/cli,docker/cli,redpanda/cli,cpuguy83/cli,redpanda/cli