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.
We could just add an example of usage, or some text that says "one or more..." somehwere. (Or skip it--just a suggestion.)