Add dedicated help for `using` directives

[Gedochao]

Is your feature request related to a problem? Please describe.
Scala CLI currently doesn't clearly explain what using directives are and how they should be used anywhere in its help messages.
We should give some info to the users from the command line, or at least point them to the appropriate online doc.

Describe the solution you'd like
similarly to flags like --help-native and --help-js, we could have a separate --help-using-directives which would print relevant help to the console.

Describe alternatives you've considered
We could always link to this doc in other help outputs... but it'd be nice to at least cover some basics in-terminal, I think.