[whatwg/fetch] Document which request fields are required (#949)

It's not clear from https://fetch.spec.whatwg.org/#concept-request which fields another specification needs to set in order to [fetch](https://fetch.spec.whatwg.org/#concept-fetch) the request. I believe https://www.w3.org/TR/appmanifest/#obtaining forgets to set the client as a result.

I think the required fields are:

* [client](https://fetch.spec.whatwg.org/#concept-request-client) (because [fetch](https://fetch.spec.whatwg.org/#concept-fetch) uses "request’s client’s global object" without checking whether the client is null)
* [URL](https://fetch.spec.whatwg.org/#concept-request-url)

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