add doc for Node.append

tools/dom/docs/docs.json

 {
   "dart.dom.html": {
     "CanvasGradient": {
       "comment": [
         "/**",
         " * An opaque canvas object representing a gradient.",
         " *",
         " * Created by calling [createLinearGradient] or [createRadialGradient] on a",
         " * [CanvasRenderingContext2D] object.",
         " *",
@@ skipping 309 matching lines @@
         " *",
         " * A <menu> element represents an unordered list of menu commands.",
         " *",
         " * See also:",
         " *",
         " *  * [Menu Element](https://developer.mozilla.org/en-US/docs/HTML/Element/menu) from MDN.",
         " *  * [Menu Element](http://www.w3.org/TR/html5/the-menu-element.html#the-menu-element) from the W3C.",
         " */"
       ]
     },
+    "Node": {
+      "members": {
+        "appendChild": [
+          "/**",
+          "   * 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.",
+          "   */"
+        ]
+      }
+    },
     "WebSocket": {
       "comment": [
         "/**",
         " * Use the WebSocket interface to connect to a WebSocket,",
         " * and to send and receive data on that WebSocket.",
         " *",
         " * To use a WebSocket in your web app, first create a WebSocket object,",
         " * passing the WebSocket URL as an argument to the constructor.",
         " *",
         " *     var webSocket = new WebSocket('ws://127.0.0.1:1337/ws');",
@@ skipping 260 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }