`docker network prune` prunes unused networks, including overlay ones.
`docker system prune` also prunes unused networks.
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Better formatting for usage template.
Group commands in usage to management/operation commands.
Remove the word Docker from the description of management commands.
Signed-off-by: Daniel Nephin <dnephin@docker.com>