Move docs for TimedStack to just above the TimedStack class definition

I was reading through the source and thought I was reading about when the `PoolShuttingDownError` gets raised but was actually reading about how to use the `TimedStack` class. I think reorganizing the comments like this makes more sense.

The diff seems to have come out a little bit weird, what I did was move the `example` comments to just above the `TimedStack` class definiton.