text: Title: SARIMAX.predict() exog
  Body: On SARIMAX.predict(), when you have an exog but the exog is only known today and in the past, how do you predict the endog's next 12 months off just the exog and data known through today?  Is that what the SARIMAX.predict() is doing as a default?  Example, my exog is SP500 price.  I do not know what it will be tomorrow, but I want to use the values through today as an exog to predict the next 12 months forward of my endog.
  
  Or is SARIMAX.predict() wanting you to know what the future exog values will be for the 12 months into the future of the endog you are predicting?  Example, my exog is days  of the week.  I know today what day of the week things will happen on so I can fill in the future exog for the next 12 months to predict the endog for the same forward period.
  
  And if the exog value is only known for today as in the SP500 example, will SARIMAX.forecast(steps = forecast_period, exog = exog[-forecast_period:]) result in an accurate modeled prediction of the 12 months ahead?  Is it by default calculating a single step 12 month ahead forecast.  Or is it trying to multistep off the exogs which in this case wouldn't be possible without predicting those exogs 1 step ahead as well.
  
  Any help would be greatly appreciated!