document the sec= option

Document the sec= option in the exports man page.

Not done: it would be nice to have an example or two here (and not just
in the final "EXAMPLE" section, though that would be nice too).  I was
just too lazy to figure out the formatting.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>