fix(docs/user_guide): wrong link to ps:scale
docs(client): the big docstring refactor

This refactors the docstrings such that they provide more useful
information to the user.

BREAKING CHANGES: the clusters:create option used to require a couple
CLI options: `--hosts` and `--auth`. Since these "options" were
mandatory, I promoted them to standalone arguments. For example, you
used to create a cluster like so:

    $ deis clusters:create dev example.com --hosts=example.com,example2.com --auth=~/.ssh/deis

With the new syntax, it looks like:

    $ deis clusters:create dev example.com example.com,example2.com ~/.ssh/deis