Add Combobox to docs

Fixes: jupyter-widgets#2573

Author: mlucool

docs/source/examples/Widget List.ipynb

@@ -675,7 +675,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "There are several widgets that can be used to display a string value.  The `Text` and `Textarea` widgets accept input.  The `HTML` and `HTMLMath` widgets display a string as HTML (`HTMLMath` also renders math). The `Label` widget can be used to construct a custom control label."
+    "There are several widgets that can be used to display a string value.  The `Text`, `Textarea`, and `Combobox` widgets accept input.  The `HTML` and `HTMLMath` widgets display a string as HTML (`HTMLMath` also renders math). The `Label` widget can be used to construct a custom control label."
    ]
   },
   {
@@ -724,6 +724,29 @@
     ")"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "### Combobox"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "widgets.Combobox(\n",
+    "    # value='John',\n",
+    "    placeholder='Choose Someone',\n",
+    "    options=['Paul', 'John', 'George', 'Ringo'],\n",
+    "    description='Combobox:',\n",
+    "    ensure_option=True,\n",
+    "    disabled=False\n",
+    ")"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {
@@ -1175,5 +1198,5 @@
   }
  },
  "nbformat": 4,
- "nbformat_minor": 2
+ "nbformat_minor": 4
 }