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.
Users have not noticed that this syntax means "one or more" and missed this handy Deis command variant:
```console
$ deis config:set FOO=alpha BAR=beta BAZ=gamma
```
I wonder if there is some way we can point out in text that it is repeatable. (Just an idea.)