OpenZFS 7276 - zfs(1m) manpage could better describe space properties

Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7276
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d750135
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/29c6739
Closes #5549