[DOC] Confusion on Y_df and Y_fitted_df inputs for residuals based reconciliation.

[kdgutier]

The core.reconcile's method Y_df Input names is a bit misleading.

As it is suggesting the possibility of only feeding the models Y_train rather than [Y_fitted, Y_train] that residual based methods use.

It would be convenient that we change the name in future release to avoid confusions.
This is also convenient to match StatsForecast's Y_fitted_df output.

[elephaint]

The problem with renaming it to Y_fitted_df is that there are methods that only need Y_df, or only need a portion of forecast values.

See also #290.

Need to think about this.