Re: [webcomponents] Update Section 6.2 Focus Navigation to reflect TPAC discussion (#375)

@rniwa in https://github.com/w3c/webcomponents/issues/375#issuecomment-178630606 's composed tree example, as `<my-nameinput>` doesn't have `tabindex`, I guess `#search` is visited first, then dive inside `<my-nameinput>`, because in that scope there are only `<my-nameinput>` and `<input>` and even we implicitly treat `<my-nameinput>`'s scope order as `tabindex=0`.

So even if you were just a web author using the third-party `<contact-card>` component, you have to be aware that there are separate `tabindex` scopes, one in `<contact-card>` and one in the main document.
If it's `<contact-card>`, it can be imaginable that the element is probably a custom element with shadow DOM, but in the case when it is a `<div>` with a shadow DOM, it will be hard to distinguish scopes at looking at the main document source alone?

Let me think more...

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