Fix documentation of Control.Concurrent.Async.Warden.shutdown

[sol]

I changed the documentation to what the current code does.

However, personally, I think I would prefer the documented behavior:

• Provided you use withWarden, misusing this takes such deliberate action that I'm not convinced it's worth protecting against it.
• Going with the documented behavior:
  • keeps the documentation (and by extension behavior) simpler
  • allows you to remove WardenException entirely