trust sign: docs for docker trust sign
trust sign: docs for docker trust sign

Signed-off-by: Riyaz Faizullabhoy <97794a5efeea78234636d4fba0f499c15a92f5e3@docker.com>

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