Add a command to print the full docstring of a session

[ThomasLaPiana]

How would this feature be useful?

When a project has a lot of tasks, particularly tasks that developers might use as part of the development flow, it can be helpful to show a more verbose help message than the one provided via nox --list

Describe the solution you'd like

A command, maybe something like nox [--usage|-u] <session> that would print the full docstring of the session function

Describe alternatives you've considered

I wrote a hacky version of this for our own project, where usage is actually another session that prints the docstring of another session. It works, but this seems like a useful feature to have built-in

https://github.com/ethyca/fides/blob/main/noxfile.py#L30-L44

Anything else?

Thanks for making such a great tool, it's been a great replacement for Make 🙂