Add some docstring

packages/tree-extension/src/index.ts

@@ -32,8 +32,7 @@ import {
 
 import { Menu, MenuBar } from '@lumino/widgets';
 
-import { NotebookTreeWidget } from '@jupyter-notebook/tree';
-import { INotebookTree } from '@jupyter-notebook/tree';
+import { NotebookTreeWidget, INotebookTree } from '@jupyter-notebook/tree';
 
 /**
  * The file browser factory.

packages/tree/src/notebook-tree.ts

@@ -4,7 +4,13 @@ import { TabPanel } from '@lumino/widgets';
 
 import { INotebookTree } from './token';
 
+/**
+ * The widget added in main area of the tree view.
+ */
 export class NotebookTreeWidget extends TabPanel implements INotebookTree {
+  /**
+   * Constructor of the NotebookTreeWidget.
+   */
   constructor() {
     super({
       tabPlacement: 'top',

packages/tree/src/token.ts

@@ -1,6 +1,9 @@
 import { Token } from '@lumino/coreutils';
 import { TabPanel } from '@lumino/widgets';
 
+/**
+ * The INotebookTree interface.
+ */
 export interface INotebookTree extends TabPanel {}
 
 /**