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

Yes.  Just like nested browsing context, elements will be ordered first by the descending order of `tabindex` value and then the tree order.

Just as another example, if the DOM looked like:
```html
<contect-card>
    Name first: <input id="first" slot="name" tabindex="2">
    Middle name: <input id="middle" slot="name">
    Last name: <input id="last" slot="name" tabindex="1"><br>
    Email: <input id="email" tabindex="3">
</contect-card>
```
and the composed tree looked like:
```html
<contect-card>
    <dl>
        <dt>Name</dt>
        <dd><input id="first" slot="name" tabindex="2">
        ><input id="middle" slot="name">
        ><input id="last" slot="name" tabindex="1"></dd>
        <dt>Email</dt>
        <dd><input id="email" tabindex="3"></dd>
    </dl>
</contect-card>
```
instead, then the focus navigation order will be `#last`, `#first`, `#middle`, then `#email`.

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https://github.com/w3c/webcomponents/issues/375#issuecomment-178717042