updated the docstring in accordance with GH40378

Author: GivyBoy

pandas/core/frame.py

@@ -8332,6 +8332,18 @@ def update(
 1  Falcon      370.0
 2  Parrot       24.0
 3  Parrot       26.0
+
+When using ``.groupby()`` on a multiple index dataframe, do not specify both 
+``by`` and ``level``.
+The argument validation should be done in ``.groupby()``, 
+using the name of the specific index.
+
+>>> df = pandas.DataFrame({"col1": ["a", "b", "c"]})
+>>> df.index = pandas.MultiIndex.from_arrays([["a", "a", "b"], 
+...                                           [1, 2, 1]], 
+...                                           names=["x", "y"])
+>>> df.groupby(["col1", "x"]) # Fine
+>>> df.groupby("col1", level=0)  # TypeError
 """
     )
     @Appender(_shared_docs["groupby"] % _shared_doc_kwargs)