[3.12] gh-101578: [doc] mention that PyErr_GetRaisedException returns NULL when the error indicator is not set (GH-113369) (#113606)

gh-101578: [doc] mention that PyErr_GetRaisedException returns NULL when the error indicator is not set (GH-113369)
(cherry picked from commit 2849cbb53afc8c6a4465f1b3490c67c2455caf6f)

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