Fix or remove comment on out-of-bag-error for Random Forests

@mikolajr This is not a features importance-related error but online-validation error (related to not features but samples bootstraping). It allows to pick a suitable number of trees much cheaper than when running a grid search - see: see: https://scikit-learn.org/stable/auto_examples/ensemble/plot_ensemble_oob.html#sphx-glr-auto-examples-ensemble-plot-ensemble-oob-py .