code: public ReadModifyWriteRow append(
        @Nonnull String familyName, @Nonnull String qualifier, @Nonnull String value) {
      return append(familyName, ByteString.copyFromUtf8(qualifier), ByteString.copyFromUtf8(value));
    }
docs: Appends the value to the existing value of the cell. If the targeted cell is unset, it will be
  treated as containing the empty string.