docs: public FindByIndexOptions useIndex(String designDocument, String indexName) {
          assertNotNull(designDocument, "designDocument");
          assertNotNull(indexName, "indexName");
          JsonArray index = new JsonArray();
          index.add(new JsonPrimitive(designDocument));
          index.add(new JsonPrimitive(indexName));
          this.useIndex = index;
          return this;
      }