DOC: Ordering during set operation on index (pandas-dev#3123) (pandas-dev#14629)

Author: mroeschke

doc/source/indexing.rst

@@ -1467,6 +1467,10 @@ with duplicates dropped.
    idx1.symmetric_difference(idx2)
    idx1 ^ idx2
 
+.. note::
+
+   The resulting index from a set operation will be sorted in ascending order.
+
 Missing values
 ~~~~~~~~~~~~~~