`ModelSelector` docstrings reference a removed "DataFrame mode"

[chripiermarini]

src/forecasting/evaluation/model_selector.py docstrings describe two modes — a "DataFrame mode (legacy pipeline)" and a "tuple-list mode (per-destination pipeline)" — but only select_best_from_tuples exists in the current implementation.

The DataFrame-mode methods (select_best, select_best_from_dataframe, rank_models) were already removed in an earlier cleanup.

Tasks:

• Update the class/method docstrings to describe only the current tuple-list API.

[al2cibo]

Already handled at cefb3c1