Improve `Usage` section to show how to use `--help`

[Braqzen]

Summary

When running fuelup --help it tells you to run help on any sub-command. It does not tell you how to run the command.

Fix: Change usage to indicate that a user must run fuelup <SUBCOMMAND> --help instead of fuelup --help <SUBCOMMAND>

Screenshots

[adlerjohn]

This is standard Rust CLI, generated automatically from clap. All Rust CLI tools that use clap, along with Cargo itself, behave this way.

[Braqzen]

This is standard Rust CLI, generated automatically from clap. All Rust CLI tools that use clap, along with Cargo itself, behave this way.

Then they all need to be improved to be explicit. Bad standards should not be followed just because they are a standard. Furthermore, Fuel is supposed to be learning from past mistakes so unless I'm incorrect in this being ambiguous then this should be fixed.

[mitchmindtree]

This is not just a convention in Rust, but a convention in a lot of other unix-y CLI tools too.

E.g. I can do nix --help and it will show me all the commands etc like you'd expect. But I can also do nix build --help, an it will show me all the options etc for the nix build command, and so on.

That said, I agree it would be nice if tools showed that this could be done in the top-level default help output, as you're right that all these unix-y conventions aren't at all obvious to non-grey-beards and n00bs.

[JoshuaBatty]

I'm not convinced what we already have isn't sufficient. Closing.