mount_nfs: update man page description for oneopenown

A recent email discussion indicated that a large
accumulation of NFSv4 Opens was occurring on
a mount. This appears to have been caused by a
shared library within the mount being used by
several processes, such that there is always at
least one of these processes running.
A new Open was created by each process and
were not closed, since all the Opens were never
closed. This is alleviated by using the
"oneopenown" mount option.

This man page update attempts to indicate the
use of "oneopenown" for this case.

This is a content change.

Reported by: j.david.lists@gmail.com
Reviewed by: 0mp
MFC: 1 month
Differential Revision: https://reviews.freebsd.org/D28215