exports.5: Clarify that exported dirs should be local mount points

If not, then in general the entire filesystem containing the exported
directory is accessiable. This may be surprising, so try to make it
more clear.

Reviewed by: rmacklem, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44614

(cherry picked from commit 9d975e47d5a3638d4f575b2cf97e07bf22b53c7e)