Re: [w3c/webcomponents] Add Scoped CustomElementRegsitry explainer (#865)

@caridy 

> I'm still unclear on how is this different from 1 registry (the global one) vs 10 registries? can you elaborate more?

The current spec has a definitive order for upgrades. With a scoped registry associated with multiple shadow roots, there are at least two plausible orderings for which shadow roots, and thus which elements, to upgrade first: shadow-including document order, or shadow root creation order (order that shadow roots were associated with the registry).

> This should not be different to what we do today with the global registry. New entries on those registries can cause upgrades on multiple shadow roots, isn't it? Just like adding a new entry into a scoped registry can cause similar upgrades, in the same fashion, including same ordering.

We _may_ want to pick the option where we use shadow root creation order because we wouldn't have to walk the entire shadow-including document, we could just visit each associated shadow root and consider only descendants. That seems like a faster choice, though there could be an inconsistency with the global registry if we include disconnected shadow roots.

Keeping a list of shadow roots with a registry would create a cycle between shadow roots and registries. I don't know if that's something generally avoided in the DOM, which is why I'm asking for the guidance.

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