add explanation for retry policy example in node-retries notebook

docs/docs/how-tos/node-retries.ipynb

@@ -231,6 +231,13 @@
     "final_state = app.invoke({\"counter\": 0, \"result\": \"\"})\n",
     "print(final_state)"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "In this example, we define a retry policy that maps `ValueError` to the `increment_counter` function. When a `ValueError` occurs, `increment_counter` is called with the current state and the exception before the retry attempt. "
+   ]
   }
  ],
  "metadata": {