docs: update docs for tree, close: #4932

components/tree/demo/draggable.md

@@ -106,6 +106,7 @@ class Demo extends React.Component {
     });
     return (
       <Tree
+        className="draggable-tree"
         defaultExpandedKeys={this.state.expandedKeys}
         draggable
         onDragEnter={this.onDragEnter}
@@ -119,3 +120,10 @@ class Demo extends React.Component {
 
 ReactDOM.render(<Demo />, mountNode);
 ````
+
+````css
+// You can add the following CSS to your project to make draggable area bigger
+#components-tree-demo-draggable .draggable-tree .ant-tree-node-content-wrapper {
+  width: calc(100% - 18px);
+}
+````