add doc for Node.append

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 33996de0038d10fba3f8af627d411f0d34c62eb1..978750038d57be81aeb06d00dcc5f1efa232455f 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -16239,6 +16239,15 @@ class Node extends EventTarget {
   @DocsEditable
   void $dom_addEventListener(String type, EventListener listener, [bool useCapture]) native "Node_addEventListener_Callback";
 
+  /**
+   * Adds a node to the end of the child [nodes] list of this node.
+   *
+   * If the node already exists in this document, it will be removed from its
+   * current parent node, then added to this node.
+   *
+   * This method is more efficient than `nodes.add`, and is the preferred
+   * way of appending a child node.
+   */
   @DomName('Node.appendChild')
   @DocsEditable
   Node append(Node newChild) native "Node_appendChild_Callback";