[m-rev.] for post-commit review: improve getopt_io.m documentation

[Julien Fischer]

On Tue, 8 Sep 2020, Zoltan Somogyi wrote:

> There are no algorithmic changes.
>
> I will make corresponding changes to getopt.m *after* I had a chance to act
> on any suggestions raised by reviews.
>

That looks fine.

Julien.