docs: explain what docker run -a does
docs: explain what docker run -a does

This adds a bit of documentation for the `-a` flag for docker run.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)

cpuguy83/cli,redpanda/cli,redpanda/cli,thaJeztah/cli,Zebrilee/cli,docker/cli,e11137/cli,ashfall/cli,dnephin/cli,allencloud/cli,riyazdf/cli,dnephin/cli,allencloud/cli,docker/cli,riyazdf/cli,thaJeztah/cli,cpuguy83/cli,ashfall/cli,e11137/cli,Zebrilee/cli