mount_nfs.8: Note that NFSv4 requires unique /etc/hostid's

Recent problems related to NFSv4 mounts has been traced
to multiple NFSv4 clients using the same /etc/hostid
(or kern.hostuuid, if you prefer).

This patch adds a sentence to the man page noting that
clients must have unique /etc/hostid's.

This is a content change.

Reviewed by: gbe (manpages)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D36392