nfsd.8: Add a short paragraph w.r.t. NFSv3 vs NFSv4 setup

The nfsd can be configured to support NFSv3, NFSv4 or both
of them.

This patch adds a short paragraph to nfsd.8 to explain this.

This is a content change.

(cherry picked from commit 4f184fd35d81bbd85284d47d2a65aeece67e87d4)