Change \otsref to \otsoref and modernize comment.

(In the OTS SVN tree, r21800 adjusts for this change.)

The original motivation for this change was to avoid confusion with
\otsparenref and some other related commands that I'm about to add.
But then when I looked more closely, I realized that the documentation
for \otsref (now \otsoref) was quite out-of-date too, so I updated it.