[css-ui] State that `pointer-events: none` on frame prevents hit-testing its contents

https://drafts.csswg.org/css-ui-4/#valdef-pointer-events-none defines pointer-events: none as

Hit-testing acts as if the boxes generated by the element (see [CSS-DISPLAY-3]) were not there, effectively causing the element behind the pointer-events: none element to become the target of hit-testing based events instead.

This should make it explicit that if you have something like

<button>You can click me</button><br>
<iframe style="pointer-events: none" srcdoc="<button>You can't click me</button>"></iframe>

Then hit-testing will skip the button inside the iframe, even if it has pointer-events: auto (CSS properties don't inherit into iframes).

We already have interoperability among Blink, WebKit and Gecko. They behave as desired.

And this is important for inert: if the iframe becomes inert, it gets pointer-events: none per https://github.com/w3c/csswg-drafts/issues/6685#issuecomment-930305697, and this should prevent interaction with its contents, even if they aren't marked as inert.

Start with the CSS UI 4 definition of pointer-events: none and its hit-testing text linked in the issue, then review the related inert discussion in issue #6685. Update the specification to state that hit-testing skips an iframe's contents when the iframe has pointer-events: none, and verify that the wording reflects the interoperable Blink, WebKit, and Gecko behavior described here.