Adaptive docs could do with a bit more details

I've been piecing together an auto-scaling `dask` cluster on AWS, using `adaptive` and bits from `dask_ec2`. It would be really useful to know what the semantics of the `scale_up` and `scale_down` calls are, especially if `scale_up` should return immediately, block, return a future or something else. It isn't clear from the examples (so I'm currently reading the sources).

My main concern here is what happens when long blocking call are made inside `scale_up` or `scale_down` - for instance bringing up new AWS instances?

I'm guessing that the best way to approach this is to return a future by using `tornado.gen.coroutine`.