[webcomponents] Add a non-normative note stating elements in shadow trees aren't exposed as named properties (#304)

I think the following text in the spec
> Window object named properties [HTML] must access the nodes in the document tree.
is meant to clarify that elements in shadow trees aren't exposed as named properties on `Window`.  Perhaps we want to add an informative note stating that explicitly.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/304