Docs: a brief note in the sets tutorial about order (pythonGH-145984)

(cherry picked from commit 4f5e798)

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>
Docs: a brief note in the sets tut about order

Author: nedbat

Doc/tutorial/datastructures.rst

@@ -454,6 +454,9 @@ Curly braces or the :func:`set` function can be used to create sets.  Note: to
 create an empty set you have to use ``set()``, not ``{}``; the latter creates an
 empty dictionary, a data structure that we discuss in the next section.
 
+Because sets are unordered, iterating over them or printing them can
+produce the elements in a different order than you expect.
+
 Here is a brief demonstration::
 
    >>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}