mountd.8: Update the man page for -N and -s

Commit 7c2901b08b4e added new command line options "-N" and "-s".
This patch updates the man page for this.

This is a content change.

Reviewed by: Alexander Ziaee <concussious.bugzilla@runbox.com> (manpages)
Differential Revision: https://reviews.freebsd.org/D45118