docs: fix message transformer docstrings (#23264)

libs/core/langchain_core/messages/utils.py

@@ -296,6 +296,7 @@ def _runnable_support(func: Callable) -> Callable:
                 partial(func, **kwargs), name=getattr(func, "__name__")
             )
 
+    wrapped.__doc__ = func.__doc__
     return wrapped