DOC: Update sort_index docs (#31898)

Author: dsaxton

Diff for: pandas/core/frame.py

@@ -4732,8 +4732,9 @@ def sort_index(
             and 1 identifies the columns.
         level : int or level name or list of ints or list of level names
             If not None, sort on values in specified index level(s).
-        ascending : bool, default True
-            Sort ascending vs. descending.
+        ascending : bool or list of bools, default True
+            Sort ascending vs. descending. When the index is a MultiIndex the
+            sort direction can be controlled for each level individually.
         inplace : bool, default False
             If True, perform operation in-place.
         kind : {'quicksort', 'mergesort', 'heapsort'}, default 'quicksort'

Diff for: pandas/core/series.py

@@ -2950,11 +2950,11 @@ def sort_index(
         self,
         axis=0,
         level=None,
-        ascending=True,
-        inplace=False,
-        kind="quicksort",
-        na_position="last",
-        sort_remaining=True,
+        ascending: bool = True,
+        inplace: bool = False,
+        kind: str = "quicksort",
+        na_position: str = "last",
+        sort_remaining: bool = True,
         ignore_index: bool = False,
     ):
         """
@@ -2969,8 +2969,9 @@ def sort_index(
             Axis to direct sorting. This can only be 0 for Series.
         level : int, optional
             If not None, sort on values in specified index level(s).
-        ascending : bool, default true
-            Sort ascending vs. descending.
+        ascending : bool or list of bools, default True
+            Sort ascending vs. descending. When the index is a MultiIndex the
+            sort direction can be controlled for each level individually.
         inplace : bool, default False
             If True, perform operation in-place.
         kind : {'quicksort', 'mergesort', 'heapsort'}, default 'quicksort'