Clarify child templates

examples/user_guide/Custom_Components.ipynb

@@ -358,6 +358,13 @@
     "select"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "The loop body can declare any number of HTML tags to add for each child object, e.g. to add labels or icons, however the child object (like the `${option}`) must always be wrapped by an HTML element (e.g. `<option>`) which must declare an `id`. Depending on your use case you can wrap each child in any HTML element you require, allowing complex nested components to be declared."
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},