The wording in the exportfs man page can be a bit confusing, implying

that "exportfs -u :/foo" will unexport /foo from all hosts, which it won't.

This patch cleans that wording up and adds a couple examples on how to
unexport directories

Signed-off-by: Steinar H. Gunderson <sesse@debian.org>
Signed-off-by: Steve Dickson <steved@redhat.com>