Document unique statistical codes in release notes

Response Code: HTTP/1.1 500 Internal Server Error
Success code from [100‥599]
[Pipeline] echo
500
Tenant operation failed for module mod-inventory-storage-20.0.1: SQL error
     DO $do$
    BEGIN
      PERFORM rmb_internal_index(
      'statistical_code', 'statistical_code_name_idx_unique', 'ADD',
      'CREATE UNIQUE INDEX IF NOT EXISTS statistical_code_name_idx_unique ON fs09000000_mod_inventory_storage.statistical_code '
      || $rmb$(lower(f_unaccent(jsonb->>'name')))$rmb$
      );
    END $do$;