Attempt at clarifying request language

Not thrilled with this ... but it's a starting point:

- Calls from a `Subscriber` to `Subscription` such as `Subscription.request(int n)` must be dispatched asynchronously (separate thread, event loop, trampoline, etc) so as to not cause a StackOverflow since `Subscriber.onNext` -> `Subscription.request` -> `Subscriber.onNext` can recurse infinitely.