code: public EqualsVerifierApi<T> withNonnullFields(String... fields) {
          List<String> fieldsAsList = Arrays.asList(fields);
          nonnullFields.addAll(fieldsAsList);
          Validations.validateFieldNamesExist(type, fieldsAsList, actualFields);
          Validations.validateNonnullFields(nonnullFields, warningsToSuppress);
          return this;
      }
docs: Signals that all given fields can never be null, and {@code
  EqualsVerifier} therefore doesn't have to verify that proper null checks
  are in place for these fields.
  
  This can be used instead of {link #suppress(Warning...)}, which provides
  the same behaviour for all fields, when only some fields are never null
  but others are.
  
  @param fields Fields that can never be null.
  @return {@code this}, for easy method chaining.