Help for Args, like with Opt

[parnmatt]

Currently clara::Arg(filenames, "filename"s)("input filenames"s) does not produce an entry in the help; it's is common for a brief description to appear

usage:
    NAME [<filename> ... ] options

    where <filename> ...     input filenames
    where options are:
    ETC

or something similar.

Frankly, I also find it strange that the options appear after the arguments, rather than before, as is fairly standard.