Document `Py_AddPendingCall()` change with subinterpreters in 3.12 (GH-139117)

Prior to 3.9, Py_AddPendingCall() would always run pending calls in the main interpreter, but then each interpreter got their own ceval state, and they were scheduled for any interpreter. In GH-104813, this was undone, so Py_AddPendingCall() would always schedule for the main interpreter.