text: Title: Clarify intended behavior of model.backtest and model.historical_forecast with multiple series and allow training on multiple series per fold when backtesting
  Body: **Is your feature request related to a current problem? Please describe.**
  It is unclear from the documentation that `model.historical_forecasts` and `model.backtest` will only ever train on a single series at a time if multiple series are provided, in contrast to `model.fit` which will train on every series provided. 
  
  **Describe proposed solution**
  Add a Boolean keyword argument to `model.backtest` and `model.historical_forecast` to train on all "past" samples in each series for each fold of backtesting. 
  
  **Describe potential alternatives**
  Clarify the documentation and possibly provide an example of how to achieve the proposed functionality. 