Node.ownerDocument should not always be (Document | null)

[rod24574575]

lib.dom.ts defines Node.ownerDocument as (Document | null):
https://github.com/microsoft/TypeScript/blob/master/lib/lib.dom.d.ts#L11033

But according to spec, only Document.ownerDocument should be null, while other Node should just be Document.
https://developer.mozilla.org/en-US/docs/Web/API/Node/ownerDocument