Print consistent help with options and commands

These commands now all output the same thing:

 - docker
 - docker help
 - docker --help
 - docker -h

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Add instructions on how to get help on commands

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Consolidate documentation for -H option

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #7957 from bfirsh/consistent-help-messages

Consistent help messages