[whatwg/fetch] Add a diagram of the call graph for the fetch algorithms (#646)

@tyoshino made [a presentation](https://docs.google.com/presentation/d/1eUtpVsuoPPPLzl6ccNoaUH9Rwmd63hgCboVh-gDLkiI/edit?usp=sharing) about the Fetch Standard for the Chrome networking team. In it he had [an amazing diagram](https://docs.google.com/presentation/d/1eUtpVsuoPPPLzl6ccNoaUH9Rwmd63hgCboVh-gDLkiI/edit?ts=5a373341#slide=id.g2c389e7de3_0_17) that we should add to the spec, showing the call graph of the different algorithms.

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