docs: add --password-stdin
docs: add --password-stdin

also add an example of usage

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

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