Re: [fetch] Clarify main fetch recursive invocation from redirect-fetch#217 (#218)

Thank you @danlrobertson and @Manishearth. This is a great clarification. 

Committed as https://github.com/whatwg/fetch/commit/88fb8d48f248396cb9c1876d0f13e682519cffa9. I ended up making a few more tweaks to fit the overall editorial style and I also added both your names to the acknowledgments section. I didn't want to get into too much nitpicking for a first PR.

(I'm not sure if you generated `Overview.html` or not since I got a different date, but if you didn't, feel free to leave it out of your next PR. I can fix such things when landing the change.)

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