Small documentation fix for MultiIndex.sortlevel (pandas-dev#15345)

* doc fix for return values of MultiIndex.sortlevel

* MultiIndex.sortlevel docs improved after feedback

Author: J535D165

pandas/indexes/multi.py

@@ -1399,7 +1399,11 @@ def sortlevel(self, level=0, ascending=True, sort_remaining=True):
 
         Returns
         -------
-        sorted_index : MultiIndex
+        sorted_index : pd.MultiIndex
+            Resulting index
+        indexer : np.ndarray
+            Indices of output values in original index
+
         """
         from pandas.core.groupby import _indexer_from_factorized