Use the usage output of `--help` for each subcommand, to make
sure all flags/options are included on the page, and to make
it easier to keep docs in sync.
Note that the usage output is only used when reading these
docs on GitHub; docs.docker.com only consumes the "description"
and "example" sections (when present), and generates flag information
and usage output from source
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
