Add pointers to create_react_agent in how-tos where appropriate (#464)

examples/async.ipynb

@@ -10,7 +10,15 @@
     "In this example we will build a ReAct agent with native [async](https://docs.python.org/3/library/asyncio.html) implementations of the core logic. When Chat Models have async clients, this can give us some nice performance improvements if you\n",
     "are running concurrent branches in your graph or if your graph is running within a larger web server process.\n",
     "\n",
-    "In general, you don't need to change anything about your graph to add `async` support. That's one of the beauties of [Runnables](https://python.langchain.com/docs/expression_language/interface/). "
+    "In general, you don't need to change anything about your graph to add `async` support. That's one of the beauties of [Runnables](https://python.langchain.com/docs/expression_language/interface/). \n",
+    "\n",
+    "\n",
+    "<div class=\"admonition tip\">\n",
+    "    <p class=\"admonition-title\">Note:</p>\n",
+    "    <p>\n",
+    "        In this how-to, we will create our agent from scratch to be transparent (but verbose). You can accomplish similar functionality using the <code>create_react_agent(model, tools=tool)</code> (<a href=\"https://langchain-ai.github.io/langgraph/reference/prebuilt/#create_react_agent\">API doc</a>) constructor. This may be more appropriate if you are used to LangChain’s <a href=\"https://python.langchain.com/v0.1/docs/modules/agents/concepts/#agentexecutor\">AgentExecutor</a> class.\n",
+    "    </p>\n",
+    "</div>    "
    ]
   },
   {