The manual only describes the steady-state DC voltage as plateau. It also shows an example of a waveform that does not have a steady-state voltage in which the waveform does not have any section that is horizontal. I'm pretty sure that the steady-state voltage is where the waveform is nearly perfectly horizontal for some amount of time and within some tolerance. The exact time and tolerance thresholds are not specified.
Actually, I have never worried about this because I see little value in the SI overshoot measurement. We care about overshoot because it stresses the IC when the overshoot is beyond the IC rail voltages. Therefore, it really only makes sense to measure the rail overshoot. The existence of SI overshoot may be an indicator that the SI of the net could be improved, but its actual value is not important, I think.