[3.12] gh-121542: Document trailing newline behavior in `set_content()` (GH-121543) (#128996)

gh-121542: Document trailing newline behavior in `set_content()` (GH-121543)
(cherry picked from commit fba475ae6f932d0aaee6832b4102b2d4c50df70f)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Yizheng Meng <dev@rapidcow.org>