add doc to JigsawSetupDialog::validateSigmaValue()

  }

  /**

   * Validates the a priori sigma values for a given QTableWidgetItem.

   *

   * This will only validate items in the a priori sigma column. Valid a priori sigma values are

   * "FREE" or any positive double. Items that are invalid will be marked with a red background

   * color.

   *

   * @param QTableWidgetItem* Pointer to the QTableWidgetItem to be validated.

   */

  void JigsawSetupDialog::validateSigmaValue(QTableWidgetItem *item) {

    // Only validate the "a priori sigma" column

    if (item->column() != 3) {