Improve documentation, document common ZFS operations, provide more refe...

...rences

This makes the `godoc` output much more informative, and provides links to quick references such as the FreeBSD ZFS manual, etc.

Please feel free to comment and give suggestions if I added any incorrect information.

I also removed the unused `prepend()` function, and made `Dataset.Snapshots()` call to the top-level `Snapshots()` function for less code redundancy.