add admonition to custom llm guide

docs/docs/how_to/custom_llm.ipynb

@@ -9,10 +9,16 @@
     "\n",
     "This notebook goes over how to create a custom LLM wrapper, in case you want to use your own LLM or a different wrapper than one that is supported in LangChain.\n",
     "\n",
-    "Wrapping your LLM with the standard `LLM` interface allow you to use your LLM in existing LangChain programs with minimal code modifications!\n",
+    "Wrapping your LLM with the standard `LLM` interface allow you to use your LLM in existing LangChain programs with minimal code modifications.\n",
     "\n",
     "As an bonus, your LLM will automatically become a LangChain `Runnable` and will benefit from some optimizations out of the box, async support, the `astream_events` API, etc.\n",
     "\n",
+    ":::caution\n",
+    "You are currently on a page documenting the use of [text completion models](/docs/concepts/text_llms). Many of the latest and most popular models are [chat completion models](/docs/concepts/chat_models).\n",
+    "\n",
+    "Unless you are specifically using more advanced prompting techniques, you are probably looking for [this page instead](/docs/how_to/custom_chat_model/).\n",
+    ":::\n",
+    "\n",
     "## Implementation\n",
     "\n",
     "There are only two required things that a custom LLM needs to implement:\n",