[3.13] gh-120496: Add a note about iterator thread-safe (gh-120685) (#120706)

gh-120496: Add a note about iterator thread-safe (gh-120685)
(cherry picked from commit 7e189aed6450c0aebcdd6ff6aefe29e2f45b173a)

Co-authored-by: Donghee Na <donghee.na@python.org>