"[T]he methods are simpler ways for plans to: [1] Disregard reductions of adjustable benefits and suspensions of nonforfeitable benefits in determining the plan's unfunded vested benefits for purposes of calculating withdrawal liability; [2] Disregard certain contribution increases if the plan is using the presumptive, modified presumptive, or rolling-5 method for purposes of determining the allocation of unfunded vested benefits to an employer; and [3] Disregard certain contribution increases for purposes of determining an employer's annual withdrawal liability payment."