jail.8: Add creating a jail from distribution set

Approved by: re (cperciva)
PR: 200803
MFC after: 3 days
Reviewed by: jamie
Differential Revision: https://reviews.freebsd.org/D53631

(cherry picked from commit d59903481dbaef9ef7e91ea4945d2355ae17c53a)
(cherry picked from commit 5d7cd576d99b99f460263d93929828d584d8f80c)