mount_nfs.8: Update the man page for commit a145cf3f73c7

The NFSv4 client now uses the highest minor version of NFSv4
by default instead of minor version 0, for NFSv4 mounts.
The "minorversion" mount option may be used to override this default.

This patch updates the man page to reflect this change. While here,
fix nfsstat(8) to be nfsstat(1).

Reviewed by: otis
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30768