Commit the latest patch from Allan Jude with some minor additional edits.

Submitted by:	Allan Jude <freebsd@allanjude.com>
Clarify the introduction and trim some redundancy in the "three goals"
section.
Provide a big fat warning message for those people who think disabling
checksums on a ZFS pool is a good idea.
Update the example for custom (user-defined) properties:
- provide a pool name as the example would not work with more than one pool
- sprinkle <replaceable>'s over the strings the user can define
- show how to remove these properties again