gh-137740: Clarify __del__ invocation mechanism in reference counting

Doc/extending/extending.rst

@@ -1084,7 +1084,12 @@ references to all its items, so when item 1 is replaced, it has to dispose of
 the original item 1.  Now let's suppose the original item 1 was an instance of a
 user-defined class, and let's further suppose that the class defined a
 :meth:`!__del__` method.  If this class instance has a reference count of 1,
-disposing of it will call its :meth:`!__del__` method.
+disposing of it will call its :meth:`!__del__` method. Internally,
+:c:func:`PyList_SetItem` calls :c:func:`Py_DECREF` on the replaced item,
+which invokes replaced item's corrresponding :c:member:`~PyTypeObject.tp_dealloc` function (i.e :c:func:`subtype_dealloc` in case of Python
+class instance). During deallocation, :c:func:`subtype_dealloc` calls :c:member:`~PyTypeObject.tp_finalize`,
+which is mapped to the :meth:`!__del__` method for class instances (see :pep:`442`).
+This entire sequence happens synchronously within the :c:func:`PyList_SetItem` call.
 
 Since it is written in Python, the :meth:`!__del__` method can execute arbitrary
 Python code.  Could it perhaps do something to invalidate the reference to