DOC: Clarification of groupby(by=) argument

pandas/core/generic.py

@@ -5092,14 +5092,15 @@ def groupby(self, by=None, axis=0, level=None, as_index=True, sort=True,
 
         Parameters
         ----------
-        by : mapping, function, str, or iterable
+        by : mapping, function, label, or list of labels
             Used to determine the groups for the groupby.
             If ``by`` is a function, it's called on each value of the object's
             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 an ndarray is passed, the
-            values are used as-is determine the groups. A str or list of strs
-            may be passed to group by the columns in ``self``
+            values are used as-is determine the groups. A label or list of
+            labels may be passed to group by the columns in ``self``. Notice
+            that a tuple is interpreted a (single) key.
         axis : int, default 0
         level : int, level name, or sequence of such, default None
             If the axis is a MultiIndex (hierarchical), group by a particular