DOC: Clarify groupby axis operations (#62853)

Author: justmhie

doc/source/user_guide/groupby.rst

@@ -137,7 +137,7 @@ We could naturally group by either the ``A`` or ``B`` columns, or both:
 
  ``df.groupby('A')`` is just syntactic sugar for ``df.groupby(df['A'])``.
 
-The above GroupBy will split the DataFrame on its index (rows). To split by columns, first do
+DataFrame groupby always operates along axis 0 (rows). To split by columns, first do
 a transpose:
 
 .. ipython::

pandas/core/frame.py

@@ -9430,7 +9430,7 @@ def groupby(
  index. If a dict or Series is passed, the Series or dict VALUES
  will be used to determine the groups (the Series' values are first
  aligned; see ``.align()`` method). If a list or ndarray of length
- equal to the selected axis is passed (see the `groupby user guide
+ equal to the number of rows is passed (see the `groupby user guide
  <https://pandas.pydata.org/pandas-docs/stable/user_guide/groupby.html#splitting-an-object-into-groups>`_),
  the values are used as-is to determine the groups. A label or list
  of labels may be passed to group by the columns in ``self``.