DOC: mention implicit sorting in docstrings

Author: toobaz

pandas/core/frame.py

@@ -3278,6 +3278,7 @@ def stack(self, level=-1, dropna=True):
         DataFrame (or Series in the case of an object with a single level of
         column labels) having a hierarchical index with a new inner-most level
         of row labels.
+        The level involved will automatically get sorted.
 
         Parameters
         ----------
@@ -3317,7 +3318,8 @@ def unstack(self, level=-1):
         a DataFrame having a new level of column labels whose inner-most level
         consists of the pivoted index labels. If the index is not a MultiIndex,
         the output will be a Series (the analogue of stack when the columns are
-        not a MultiIndex)
+        not a MultiIndex).
+        The level involved will automatically get sorted.
 
         Parameters
         ----------

pandas/core/series.py

@@ -1884,7 +1884,8 @@ def reorder_levels(self, order):
 
     def unstack(self, level=-1):
         """
-        Unstack, a.k.a. pivot, Series with MultiIndex to produce DataFrame
+        Unstack, a.k.a. pivot, Series with MultiIndex to produce DataFrame.
+        The level involved will automatically get sorted.
 
         Parameters
         ----------