Update the first Hydra example for clarity

 Revised the first parallel request example code (based on a previous version) to clarify that the third request is run in the context of the first request, and so is dependant on the result of the first request before it can be run, explaining the use of the callback.

Also reversed the order of the callback and queue in the third example, as the current code, if used in a callback to add a request to an already running queue, could result in a theoretical (if unlikely) race condition.
And I don't mind if you don't change it :)  as the code in its current form is safe. I was thinking of a scenario where this code sample could be used inside a callback for a running queue, and the request could then (theoretically) complete before the callback is defined. Defining the callback before queuing the request would prevent that. I'll leave it out of the PR and submit a separate one if you think there's any benefit. I should have done it that way in the first place.