Aloha all,

The attached patch initializes the shortn variable, uses it to add the
optional argument to \caption in two places where this wasn't
implemented, and adds a bit of documentation.

Note that the shortn variable in org-mode is already used to add an
optional argument to \caption in one case.  The patch enables this
behavior for other captions, as well.

Note also, that if the shortn variable isn't initialized nil, the
optional argument to one \caption is recycled to the next \caption
indefinitely until shortn is reset.

Thanks to Nick Dokos for explaining why an earlier patch of mine didn't
show up on patchwork (wrong MIME type, evil mail client).  This one is
sent from gnus with crossed fingers.

All the best,
Tom