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

Author: savannahostrowski

Doc/c-api/typeobj.rst

@@ -1034,7 +1034,8 @@ and :c:data:`PyType_Type` effectively act as defaults.)
       the type, and the type object is INCREF'ed when a new instance is created, and
       DECREF'ed when an instance is destroyed (this does not apply to instances of
       subtypes; only the type referenced by the instance's ob_type gets INCREF'ed or
-      DECREF'ed).
+      DECREF'ed). Heap types should also :ref:`support garbage collection <supporting-cycle-detection>`
+      as they can form a reference cycle with their own module object.
 
       **Inheritance:**