The `ForceDelete` parameter was getting sent to the upstream API call,
but only after we had already finished draining instances from
Terraform, so it was a moot point by then.
This fixes that by skipping the drain when force_delete is true, and it
also simplifies the field config a bit:
* set a default of false to simplify the logic
* remove `ForceNew` since there's no need to replace the resource to
flip this value
* pull a detail comment from code into the docs
a811a72f11