OpenZFS 8070 - Add some ZFS comments

OpenZFS 8070 - Add some ZFS comments

Authored by: Alan Somers <asomers@gmail.com>
Reviewed by: Yuri Pankov <yuri.pankov@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: bunder2015 <omfgbunder@gmail.com>
OpenZFS-issue: https://www.illumos.org/issues/8070
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/40713f2
Closes #6160