[streams] Document globals created by this spec (#254)

Partially due to our failure to use WebIDL (#45), it isn't actually documented how `ReadableStream` and friends are exposed.

We should add a section on this at least. Mainly because ExclusiveStreamReader should probably *not* be globally exposed, even though it has a constructor.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/254