[w3c/uievents] Document the cancelability of wheel events (#282)

Touch events are documented to have a behavior where if all the listeners are `passive`, then the event has `cancelable=false`:
https://w3c.github.io/touch-events/#cancelability which links to https://dom.spec.whatwg.org/#observing-event-listeners

Blink seems to also apply this same logic to wheel events, but that is not described in https://w3c.github.io/uievents/. It probably should be.

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