gobject: Document that classes/objects/interfaces are zero-filled

On initialisation, GObject guarantees to zero-fill
class/object/interface structures. Document this so people don’t spend
forever writing:
    my_object->priv->some_member = NULL;
    my_object->priv->some_other_member = NULL;

https://bugzilla.gnome.org/show_bug.cgi?id=729167