[whatwg/fetch] Consider splitting `document` into `document` and `nested-document`. (#755)

As [discussed in the context of `Sec-Metadata`](https://github.com/w3ctag/design-reviews/issues/280#issuecomment-394598257), it might be useful to clearly distinguish between top-level and nested navigation requests by splitting the `document` destination into `document` and `nested-document`.

@annevk notes that this might be a breaking change, as folks might be relying on the existing `document` destination that's exposed on `Request` objects. @wanderview, @jakearchibald: WDYT?



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