[3.12] gh-116935: Document that heap types need to support garbage collection (GH-118021) (GH-118092)

gh-116935: Document that heap types need to support garbage collection (GH-118021)
(cherry picked from commit 5d544365742a117027747306e2d4473f3b73d921)

Co-authored-by: Savannah Ostrowski <savannahostrowski@gmail.com>