[3.10] gh-96746: Docs: Clear up Py_TPFLAGS_DISALLOW_INSTANTIATION inheritance (GH-99002) (GH-99213)

gh-96746: Docs: Clear up Py_TPFLAGS_DISALLOW_INSTANTIATION inheritance (GH-99002)

The flag is not inherited, but its effect -- a NULL tp_new -- is.

Drop hints for people who come here wanting to “disallow instantiation”.
(cherry picked from commit 1438b779971605e516bd0a4051a704d6ffbbd58d)

Co-authored-by: Petr Viktorin <encukou@gmail.com>