[3.14] gh-141004: Document `Py_ARRAY_LENGTH` (GH-141601) (GH-141895)

gh-141004: Document `Py_ARRAY_LENGTH` (GH-141601)
(cherry picked from commit e0773eacb7e660b4fd6bd702b0a119e25ab3b012)

Co-authored-by: Yashraj <yashrajpala8@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>