Improve the `--help` message to indicate subcommands also have help info

[bskinn]

Small change to highlight to users that subcommand help is also available:

---

Originally posted by @pawamoy in #279 (comment):

I generally print something like this myself:

usage: griffe [GLOBAL_OPTS...] COMMAND [COMMAND_OPTS...]

Signatures for entire Python programs.

Global options:
  -h, --help  Show this help message and exit. Commands also accept the -h/--help option.

Commands:
  COMMAND
    dump      Load package-signatures and dump them as JSON.
    check     Check for API breakages or possible improvements.

Not sure if it's more visible 🤷