[webcomponents] Type extensions in SVG use shadow trees--specify, or add a note if no specification is necessary (#335)

SVG use allows an SVG subtree to be instanced multiple times. As far as I am aware, an implementation would not necessarily have to implement those instances as full-blown nodes; nor are the nodes exposed. It would be good to clear up what happens if a Custom Element is instanced into an SVG shadow tree.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/335