docs: def generate_statistics_subparser(subparsers):
      """Adds a sub-command parser to `subparsers` to generate statistics
      from a set of results."""
      parser = subparsers.add_parser(
          'stats', description=constants.STATISTICS_DESCRIPTION,
          formatter_class=ParagraphFormatter, help=constants.STATISTICS_HELP)
      parser.set_defaults(func=generate_statistics)
      utils.add_common_arguments(parser)
      utils.add_corpus_arguments(parser)
      parser.add_argument('results', help=constants.STATISTICS_RESULTS_HELP,
                          metavar='RESULTS')