adduser: Better document ZFS dataset creation.

MFC after: 3 days
PR: 280873
Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D46316
Approved by: re (cperciva)

(cherry picked from commit 9ff2ebd92891b6953bbe44c6d6a6d8bc31f5139f)
(cherry picked from commit 8398c81b7c28ea315cb98405accb701502e3636a)