Re: [w3c/uievents] Wheel event groups are not defined (Issue #338)

## User Impact

Consider the case where a site has a large map [like this site](https://weather.gc.ca/map_e.html?layers=radar&zoom=7&center=49.22362589%2C-90.26372876). The large map is a large element that has a wheel event listener that will `preventDefault`. Without wheel event groups (e.g. Firefox Release or Firefox Beta) a pan gesture or wheel scroll that starts immediately above the large map element will halt and cause the map to start zooming out as soon as the mouse is within the map element's bounds. This can cause sites that use `preventDefault` on wheel events like this to be somewhere between annoying to use to completely unusable if the element is large enough.

## Suggested Change

Blink and WebKit already group wheel events that occur within a given time without another user action like a mousemove, and I'm in the progress of getting this added to Gecko. It would be nice to document this in the [event-type-wheel](https://w3c.github.io/uievents/#event-type-wheel) section. Similar to [this section](https://github.com/w3c/uievents/blob/6e9966a37944851948923a1c1ef904cbc35008a4/sections/event-types.txt#L2246-L2250) that describes the behavior of "some user agents" in a non-normative manner. It might be helpful to start with something non-normative event though this is already the case in Blink, Webkit, and (hopefully soon) Gecko.

Note: I'm largely interested in some sort of change like this to ensure that I'm not going rogue with the changes I'm making to Gecko 😄.

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