[css-scroll-snap-2] Add SnapEvent definition

SnapEvent defines the interface of scroll-snap-related JavaScript
events, snapchanged and snapchanging.

Patchset 3: Add more details.

Author: DavMila

css-scroll-snap-2/Overview.bs

@@ -286,15 +286,17 @@ Snap Events {#snap-events}
 				<th><dfn for="snapchanged" event>snapchanged</dfn></th>
 				<td>{{SnapEvent}}</td>
 				<td>scroll containers</td>
-				<td>Fired at the element or {{Document}} at the end of a scroll (before {{scrollend}} events)
-					or after a <a href="https://drafts.csswg.org/css-scroll-snap-1/#re-snap">layout snap</a>.</td>
+				<td>Fired at the scrolling element or {{Document}} at the end of a scroll (before a {{scrollend}} event)
+					or after a <a href="https://drafts.csswg.org/css-scroll-snap-1/#re-snap">layout snap</a>
+					if the element that the scrolling element or Document is snapped to changed.</td>
 			</tr>
 			<tr>
 				<th><dfn for="snapchanging" event>snapchanging</dfn></th>
 				<td>{{SnapEvent}}</td>
 				<td>snap containers</td>
-				<td>Fired at the element or {{Document}} during scrolling (similar to {{scroll}} events).
-					Fired before a {{snapchanged}} event (if one fires).</td>
+				<td>Fired at the scrolling element or {{Document}} during scrolling (similar to {{scroll}} events),
+					(before a {{snapchanged}} event, if one fires) if the scrolling will lead to a change in the
+					element that the scrolling element or Document is snapped to.</td>
 			</tr>
 		</tbody>
 	</table>