Specify that both by and level should not be specified in groupby - GH40378 (#47778)

* updated the docstring in accordance with GH40378

* updated the docstring in accordance with GH40378

* updated the docstring in accordance with GH40378

* updated the docstring in accordance with GH40378

Author: GivyBoy

doc/source/user_guide/groupby.rst

@@ -345,6 +345,17 @@ Index level names may be supplied as keys.
 
 More on the ``sum`` function and aggregation later.
 
+When using ``.groupby()`` on a DatFrame with a  MultiIndex, do not specify both ``by`` and ``level``.
+The argument validation should be done in ``.groupby()``, using the name of the specific index.
+
+.. ipython:: python
+
+   df = pd.DataFrame({"col1": ["a", "b", "c"]})
+   df.index = pd.MultiIndex.from_arrays([["a", "a", "b"],
+                                        [1, 2, 1]],
+                                        names=["x", "y"])
+   df.groupby(["col1", "x"])
+
 Grouping DataFrame with Index levels and columns
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 A DataFrame may be grouped by a combination of columns and index levels by