[whatwg/fetch] Add more CORS protocol examples (#296)

See https://github.com/whatwg/fetch/files/249265/CORS.request.processing.-.Simple.docx for a couple of excellent examples by @roryhewitt, first mentioned at https://github.com/whatwg/fetch/pull/293#issuecomment-216945168.

When changed to use `fetch()` they can probably be included.

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