[3.12] Docs: Fix some semantic usages of `iterator.__iter__` (GH-130172) (GH-130544)

These references to an `__iter__` method mean `object.__iter__`, not `iterator.__iter__`.
(cherry picked from commit 4d3a7ea354a8ca7a9978f48e54399526d1b83f42)

Co-authored-by: Yuki Kobayashi <drsuaimqjgar@gmail.com>