[3.10] bpo-28737: Document when tp_dealloc should call PyObject_GC_UnTrack() (GH-29246) (GH-29249)

Objects that support garbage collection ("container" objects) should
call PyObject_GC_UnTrack() from their destructors before clearing any
fields which may point to other "container" objects.
(cherry picked from commit 35e1ff38ee67ee543d9fcb268c3552c5397f9b3f)

Co-authored-by: Sam Gross <colesbury@gmail.com>