[3.11] gh-59254: mention in open() doc that line buffering is for writing (GH-112318) (#112379)

gh-59254: mention in open() doc that line buffering is for writing (GH-112318)
(cherry picked from commit fafae08cc7caa25f2bd6b29106b50ef76c3e296f)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>