x.py setup should have more descriptive usage info

[jyn514]

rust/src/bootstrap/flags.rs

Lines 468 to 480 in f600154

	"setup" => {
	subcommand_help.push_str(
	"\n
	Arguments:
	This subcommand accepts a 'profile' to use for builds. For example:
	./x.py setup library
	The profile is optional and you will be prompted interactively if it is not given.",
	);
	}
	_ => {}
	};

does not say what it actually does. It should document that it only creates a config.toml (and optionally installs a pre-commit hook). I ran into some new contributors that were avoiding running it because they didn't know what it did.