Documenting how the harvest mode works.

https://bugs.launchpad.net/juju-core/+bug/1342729
This is almost never a good option, it has bitten folks quite a few times. Unknown and all have pathological behavior modes that have resulted in accidental environment  termination a few times for various reasons. i almost feel like this stuff should be in dev docs in juju git repo and not end user if we take as a given that juju defaults to sane behavior (destroyed).