Add explanation about View#_itemEvents.native.

src/view/View.js

@@ -1182,6 +1182,9 @@ new function() { // Injection scope for event handling on the browser
          */
         _handleMouseEvent: function(type, event, point) {
             var itemEvents = this._itemEvents,
+                // Look up hitItems, which tells us whether a given native mouse
+                // event requires an item hit-test or not, before changing type
+                // to the virtual value (e.g. mousemove -> mousedrag):
                 hitItems = itemEvents.native[type],
                 tool = this._scope.tool,
                 view = this;