mountd: Update exports.5 for commit 7c5146da1286

Commit 7c5146da1286 modified mountd so that it uses
strunvis(3) to decode directory names in exports lines.
This allows special characters, such as blanks, to be
encoded in the directory names.

This patch updates the exports.5 man page for this change.

This is a content change.

Reviewed by: karels, pauamma_gundo.com (manpages)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42163