[RFC] plugins should support a --help flag

[thaJeztah]

relates to #1661 and #1534

One thing I just noticed;

Usage:	docker [OPTIONS] COMMAND

A self-sufficient runtime for containers

...

Commands:
...
  commit                    Create a new image from a container's changes
  compose     (Docker)      Define and run multi-container applications

Run 'docker COMMAND --help' for more information on a command.

The help / usage output states that a COMMAND should handle a --help flag, so should we define that a plugin should support that flag (at least at their top-level command) as part of the "api"?

[thaJeztah]

/cc @ijc @silvin-lubecki @chris-crone

[ijc]

At the CLI level running docker «plugin» --help already DTRT and calls --help on the plugin, it's up to the plugin what to do with it, the builtin framework/helper implements it too.

We can certainly recommend they implement --help (and similarly help «command») but I think this is just a docs change, no code change needed.

[thaJeztah]

Yes, no code change is needed, but we should have it defined (or at least mentioned) as part of the "contract" for plugins.