exports(5): -maproot, -mapall: Explain the fallback on no group

Reviewed by: rmacklem (older version)
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47012

(cherry picked from commit 2ef608de90629df37c29043afc638f8ff99ecfc4)