docs: Update doc-string in base callback managers (#15885)

Update doc-strings with a comment about on_llm_start vs.
on_chat_model_start.

libs/core/langchain_core/callbacks/base.py

@@ -166,7 +166,12 @@ class CallbackManagerMixin:
         metadata: Optional[Dict[str, Any]] = None,
         **kwargs: Any,
     ) -> Any:
-        """Run when LLM starts running."""
+        """Run when LLM starts running.
+
+        **ATTENTION**: This method is called for non-chat models (regular LLMs). If
+            you're implementing a handler for a chat model,
+            you should use on_chat_model_start instead.
+        """
 
     def on_chat_model_start(
         self,
@@ -179,7 +184,13 @@ class CallbackManagerMixin:
         metadata: Optional[Dict[str, Any]] = None,
         **kwargs: Any,
     ) -> Any:
-        """Run when a chat model starts running."""
+        """Run when a chat model starts running.
+
+        **ATTENTION**: This method is called for chat models. If you're implementing
+            a handler for a non-chat model, you should use on_llm_start instead.
+        """
+        # NotImplementedError is thrown intentionally
+        # Callback handler will fall back to on_llm_start if this is exception is thrown
         raise NotImplementedError(
             f"{self.__class__.__name__} does not implement `on_chat_model_start`"
         )
@@ -308,7 +319,12 @@ class AsyncCallbackHandler(BaseCallbackHandler):
         metadata: Optional[Dict[str, Any]] = None,
         **kwargs: Any,
     ) -> None:
-        """Run when LLM starts running."""
+        """Run when LLM starts running.
+
+        **ATTENTION**: This method is called for non-chat models (regular LLMs). If
+            you're implementing a handler for a chat model,
+            you should use on_chat_model_start instead.
+        """
 
     async def on_chat_model_start(
         self,
@@ -321,7 +337,13 @@ class AsyncCallbackHandler(BaseCallbackHandler):
         metadata: Optional[Dict[str, Any]] = None,
         **kwargs: Any,
     ) -> Any:
-        """Run when a chat model starts running."""
+        """Run when a chat model starts running.
+
+        **ATTENTION**: This method is called for chat models. If you're implementing
+            a handler for a non-chat model, you should use on_llm_start instead.
+        """
+        # NotImplementedError is thrown intentionally
+        # Callback handler will fall back to on_llm_start if this is exception is thrown
         raise NotImplementedError(
             f"{self.__class__.__name__} does not implement `on_chat_model_start`"
         )
@@ -359,8 +381,9 @@ class AsyncCallbackHandler(BaseCallbackHandler):
         **kwargs: Any,
     ) -> None:
         """Run when LLM errors.
+
         Args:
-            error (BaseException): The error that occurred.
+            error: The error that occurred.
             kwargs (Any): Additional keyword arguments.
                 - response (LLMResult): The response which was generated before
                     the error occurred.