Return the usage string for the primary command.

def get_primary_command_usage(message=''):
    # type: (str) -> str
    """Return the usage string for the primary command."""
    if not settings.merge_primary_command and None in settings.subcommands:
        return format_usage(settings.subcommands[None].__doc__)
    if not message:
        message = '\n{}\n'.format(settings.message) if settings.message else ''
    doc = _DEFAULT_DOC.format(message=message)
    if None in settings.subcommands:
        return _merge_doc(doc, settings.subcommands[None].__doc__)
    return format_usage(doc)