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

Note that in the above example, if you put `tabindex` attribute in `<x-bar>`, `x-bar` itself becomes
focusable, and focus stops at `x-bar`, then the navigation gets inside `x-bar`.   The `delegatesFocus`
flag controls the behavior, whether the focus navigation slips through the element or not.
I feel like it is natural to turn it on by default for a shadow host, and can be turned of if requested, as some component may want to take focus as a whole.

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https://github.com/w3c/webcomponents/issues/375#issuecomment-179016172