Update and expand the sections on ZFS snapshots and clones.
It describes:
- what they are, what they can do and how they can be helpful,
- how to create them
- how to compare snapshots using zfs diff
- how to do rollbacks
- the .zfs directory and how to control its visibility using the ZFS property
- promoting clones to real datasets and what the origin property shows

A bunch of examples are also added to follow along with the descriptions.