[whatwg/streams] Clarify that a pull() source doesn't need to wait in pull (#1014)

When a source blocks inside `pull()`, it is not possible to cancel the ReadableStream until the promise returned by `pull()` settles. In some cases `pull()` may never return.

However, it is not necessary to block in `pull()`. In many (most?) cases, returning an already-resolved promise from `pull()` and then enqueueing later to trigger the next call to `pull()` works fine.

This should be clarified in the description of underlying source.

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