MFV r318934: 8070 Add some ZFS comments

illumos/illumos-gate@40713f2b249d289022c715107b3951055a63aef0
https://github.com/illumos/illumos-gate/commit/40713f2b249d289022c715107b3951055a63aef0

https://www.illumos.org/issues/8070

Add some ZFS comments left by various developers at different times

Reviewed by: Yuri Pankov <yuri.pankov@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Alan Somers <asomers@gmail.com>

MFC after: 1 week