[whatwg/dom] Add note that "A ShadowRoot node has no parent." (#770)

The spec:

https://dom.spec.whatwg.org/commit-snapshots/b5eac4d624b229232e8c2e347d6b532172f73439/#interface-node

already notes:
> An Attr node has no parent\. 

It'd be helpful if the note additionally contained "A ShadowRoot node has no parent.". 

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