update docstring

Author: jbrockmendel

Diff for: pandas/core/groupby/grouper.py

@@ -329,6 +329,12 @@ def _set_grouper(
         obj : Series or DataFrame
         sort : bool, default False
             whether the resulting grouper should be sorted
+        gpr_index : Index or None, default None
+
+        Returns
+        -------
+        NDFrame
+        Index
         """
         assert obj is not None