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.

(cherry picked from commit 200730f29dbc8da2aa9392b35518bf77b8899f89)