bpo-38206: Clarify tp_dealloc requirements for heap allocated types. (GH-16248)

bpo-38206: Clarify tp_dealloc requirements for heap allocated types. (GH-16248)

As mentioned in the bpo ticket, this mistake came up on two reviews:
- https://github.com/python/cpython/pull/16127GH-pullrequestreview-288312751
- https://github.com/python/cpython/pull/16071GH-pullrequestreview-287819525

Would be nice to have it documented in a more permanent place than 3.8's whatsnew entry.

https://bugs.python.org/issue38206

Automerge-Triggered-By: @encukou
(cherry picked from commit 5faff977adbe089e1f91a5916ccb2160a22dd292)

Co-authored-by: Ammar Askar <ammar@ammaraskar.com>