Clarified exception behavior of futures

[aripollak]

It was a bit surprising to me that the documentation treated concurrent.futures.wait the same as futures.result(), but the former doesn't raise an exception and the latter does. Maybe there's a better solution than this, but I thought I'd at least add some clarification.

[aripollak]

Thanks, updated with your suggestions. Would it also be useful to show a more concrete example of why someone would want to use concurrent.futures.wait, like using it with a timeout or custom return_when with multiple futures? Or would that be more confusing?

[aripollak]

There's already documentation about it in the Python standard library, so it might just help to be more explicit that our CivisFuture is not an idiosyncratic Civis API client thing.

Okay, I think that's already pretty explicit in this file, so I won't add anything else.