Improve default CLI help

[cholmes]

The message that pops up when you first try planet can use a few improvements:

planet --help
Usage: planet [OPTIONS] COMMAND [ARGS]...

  Planet API Client Parameters:     ctx -- context object     verbosity --
  user input for verbosity.

Options:
  --verbosity TEXT  Optional: set verbosity level to warning, info, or debug.
                    Defaults to warning.
  --quiet           Disable ANSI control output.
  --version         Show the version and exit.
  --help            Show this message and exit.

Commands:
  auth           Commands for working with Planet authentication
  collect        Collect a sequence of JSON descriptions into a single...
  data           Commands for interacting with the Orders API
  orders         Commands for interacting with the Orders API
  subscriptions

• The third line is a bit strange, seems like it's about the API? Planet API Client Parameters: ctx -- context object verbosity -- user input for verbosity. I'd say remove, unless there's something it's getting at I don't know?
  • maybe put verbosity last in the 'options', as that seems the least likely to be used.
• data says 'commands for interacting with the Orders API. Should be Data API.
• subscriptions has no description.