code: public static JsonStringDocument create(String id, int expiry, String content) {
          return new JsonStringDocument(id, expiry, content, 0, null);
      }
docs: Creates a {@link JsonStringDocument} which the document id, content and the expiration time.
  
  @param id the per-bucket unique document id.
  @param content the content of the document.
  @param expiry the expiration time of the document.
  @return a {@link JsonStringDocument}.