"[T]he agency called on the [DOL] to require plan sponsors to notify DOL when they implement so-called lump-sum windows for such offers, a step not currently required but also proposed ... in September 2014 by the [PBGC]. GAO also recommended clarified guidance for what information plan sponsors should provide when offering to transfer their pensions to an annuities provider, which could lead to the need for DB plan sponsors to revise their lump-sum offer information packets."