DOC: .groupby() aligns Series, accepts ndarray

closes #15789
closes #15244

Author: kernc

pandas/core/generic.py

@@ -4129,11 +4129,14 @@ def groupby(self, by=None, axis=0, level=None, as_index=True, sort=True,
 
         Parameters
         ----------
-        by : mapping function / list of functions, dict, Series, or tuple /
-            list of column names or index level names.
+        by : mapping function / list of functions, dict, Series, ndarray,
+                or tuple / list of column names or index level names or
+                Series or ndarrays
             Called on each element of the object index to determine the groups.
             If a dict or Series is passed, the Series or dict VALUES will be
-            used to determine the groups
+            used to determine the groups (the Series' values are first
+            aligned; see ``.align()`` method). If ndarray is passed, the
+            values as-is determine the groups.
         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