exports.5: Add a paragraph clarifying the use of "V4:"

The exports.5 man page is a bit of a monster.
One place of common confusion is the use of
the "V4:" line(s) for defining the location of the
root of the NFSv4 export subtree.

This patch adds a paragraph in an attempt to
clarify this.

This is a content change.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D52141