[fetch] Give guidance on when flags should be set, for other spec authors (#186)

E.g. in https://github.com/whatwg/html/pull/443/files#r48677386 I discovered that unsafe-request only applies if you can set custom headers. I think it would be very helpful if, either in the definitions of the flags or in a separate "using fetch from other specs"-type section, you explained in what scenarios another spec might want to set a flag, or leave it unset.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/186