[3.13] Docs: a brief note in the sets tutorial about order (GH-145984) (#146050)

Docs: a brief note in the sets tutorial about order (GH-145984)
(cherry picked from commit 4f5e79805ebcaa0d3ba1677694d4120a9e8f4513)


Docs: a brief note in the sets tut about order

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>