Document of fix behavior of det_cond_forecast

From https://forum.dynare.org/t/error-messages-when-building-the-forecast/22873/4. I think there is either a bug or the manual is wrong. det_cond_forecast seems to always expect three input arguments, i.e. the initial forecast date is mandatory. IN_AUX.csv UForecast.mod

Moreover, the second argument is supposed to indicate the past values of the endogenous variables. But using UForecast_3_arguments.mod I get the error message

the dseries smoothed finish at time 2023Q2 before the last period of forecast 2024Q3

which does not make sense if only past values are needed.