code: protected B fields(List<F> fields) {
          if (instance.def.fields == null) {
              instance.def.fields = new ArrayList<F>(fields.size());
          }
          instance.def.fields.addAll(fields);
          return returnThis();
      }
docs: Add fields to the text index configuration.
  
  @param fields the {@link TextIndex.Field} configurations to add
  @return the builder for chaining