rc.conf.5: Add entries for NFS

Entries for a few recently defined rc variables
were missing from rc.conf.5. This patch adds
those.

It was not obvious to me what the ordering is,
so I added them to the area where other nfsd
related variables are. I can easily move them.

I also replaced "are" with "is", since it seems to
read better.

This is a content change.

Reviewed by: debdrup
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33043