meds-dev-evaluation documentation clarity

It should be made explicitly clear in the meds-dev-evaluation cli documentation that using the predictions_dir command line argument can ONLY be used when trying to evaluate over predictions sharded across multiple subfolders within the passed path. It should also be made explicit that if you are trying to run evaluation over only a single predictions.parquet to use predictions_path instead.

While this becomes clear when running meds-dev-evaluation -- help, meds-dev should make this clear in the documentation. For example:

# Use this when predictions are sharded in subfolders
meds-dev-evaluation predictions_dir=path_with_subfolders output_dir=out_path

# Use this when you have a single Parquet file
meds-dev-evaluation predictions_path=path_to_predictions.parquet output_dir=out_path

Start by locating the CLI documentation for meds-dev-evaluation and compare it with the guidance shown by meds-dev-evaluation --help. Update the documentation to distinguish predictions_dir for predictions sharded across subfolders from predictions_path for one predictions.parquet, including the example commands; done when this distinction is explicit.