Add documentation for createTabKey.

Author: jasongrout

packages/widgets/src/tabbar.ts

@@ -1318,6 +1318,17 @@ namespace TabBar {
      */
     renderTab(data: IRenderData<T>): VirtualElement;
 
+    /**
+     * Create a stable unique id for a tab based on the title.
+     *
+     * @param data - The data to use for the tab.
+     *
+     * @returns The unique id for a tab.
+     *
+     * #### Notes
+     * This method returns a stable unique id for a tab, depending only on the
+     * title. The tab DOM `id` is set to this value.
+     */
     createTabKey(data: IRenderData<T>): string;
   }