Added helpful information on all event getters and static members.

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index e04ec31010223c88ecb96000ba283ba629af0da9..eb0e1e5ed6fd11801f861bd08e2d1fe0d483feb0 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -828,6 +828,12 @@ $endif
   }
 
 $if DART2JS
+  /**
+   * Static factory designed to expose `mousewheel` events to event
+   * handlers that are not necessarily instances of [Element].
+   *
+   * See [EventStreamProvider] for usage information.
+   */
   @DomName('Element.mouseWheelEvent')
   static const EventStreamProvider<WheelEvent> mouseWheelEvent =
       const _CustomEventStreamProvider<WheelEvent>(
@@ -846,6 +852,12 @@ $if DART2JS
     }
   }
 
+  /**
+   * Static factory designed to expose `transitionend` events to event
+   * handlers that are not necessarily instances of [Element].
+   *
+   * See [EventStreamProvider] for usage information.
+   */
   @DomName('Element.transitionEndEvent')
   static const EventStreamProvider<TransitionEvent> transitionEndEvent =
       const _CustomEventStreamProvider<TransitionEvent>(