docs: Update agents.ipynb (#31820)

docs/docs/tutorials/agents.ipynb

@@ -249,6 +249,20 @@
     "tools = [search]"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "ecbc86d8",
+   "metadata": {},
+   "source": [
+    ":::tip\n",
+    "\n",
+    "In many applications, you may want to define custom tools. LangChain supports custom\n",
+    "tool creation via Python functions and other means. Refer to the\n",
+    "[How to create tools](/docs/how_to/custom_tools/) guide for details.\n",
+    "\n",
+    ":::"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "e00068b0",