docs(client): the big docstring refactor

This refactors the client docstrings such that they provide more useful information to the user. Please note that I also refactored how `clusters:create` is parsed as part of a nitpicking (breaking backwards compatibility for the command), but I can easily revert that if the change is unnecessary. This PR should be completely backwards compatible, save the `clusters:create` change.
`<var>=<value>...` is supposed to display that, but it's missed. I'll check if there's some other way to describe that behaviour with docopt :)