Add comment explaining utility of _enableDraghover

srcjs/input_binding_fileinput.js

@@ -312,6 +312,16 @@ $.extend(fileInputBinding, {
   _zoneOf: function(el) {
     return $(el).closest("div.input-group");
   },
+  // This function makes it possible to attach listeners to the dragenter,
+  // dragleave, and drop events of a single element with children. It's not
+  // intuitive to do directly because outer elements fire "dragleave" events
+  // both when the drag leaves the element and when the drag enters a child. To
+  // make it easier, we maintain a count of the elements being dragged across
+  // and trigger 3 new types of event:
+  //
+  // 1. draghover:enter - When a drag enters el and any of its children.
+  // 2. draghover:leave - When the drag leaves el and all of its children.
+  // 3. draghover:drop - When an item is dropped on el or any of its children.
   _enableDraghover: function(el) {
     let $el = $(el),
         childCounter = 0;