[2541] Update in-code comment

src/lib/datasrc/database.cc

@@ -1397,11 +1397,8 @@ public:
                 logger.info(DATASRC_DATABASE_UPDATER_ROLLBACK)
                     .arg(zone_name_).arg(zone_class_).arg(db_name_);
             } catch (const DataSourceError& e) {
-                // We generally expect that rollback always succeeds, and
-                // it should in fact succeed in a way we execute it.  But
-                // as the public API allows rollback() to fail and
-                // throw, we should expect it.  Obviously we cannot re-throw
-                // it.  The best we can do is to log it as a critical error.
+                // See The destructor ~TransactionHolder() for the
+                // reason to catch this.
                 logger.error(DATASRC_DATABASE_UPDATER_ROLLBACKFAIL)
                     .arg(zone_name_).arg(zone_class_).arg(db_name_)
                     .arg(e.what());