code: @Override
      public StateIdsBuilder append(final String stateToken, final int document) {
          return doAppend(Collections.singletonList(new StateId(stateToken, document)));
      }
docs: Appends the specified state token and a single zero-based document index to this instance, returning
  {@code this}.
  
  @param stateToken The state token to append
  @param document The document index for the state token
  @return {@code this}