added comment for --!syntax_pg instruction

internal/dialect/dialectquery/ydb.go

@@ -41,6 +41,9 @@ func (c *Ydb) GetMigrationByVersion(tableName string) string {
 }
 
 func (c *Ydb) ListMigrations(tableName string) string {
+	// "--!syntax_pg" enables query processing with PostgreSQL-compatible syntax.
+	// YQL by design strictly forbids the execution of SELECT statements without columns from ORDER BY clause.
+	// In PostgreSQL-compatible mode, SELECT statements can be processed without columns from ORDER BY clause.
 	q := `--!syntax_pg
 	SELECT version_id, is_applied FROM %s ORDER BY tstamp DESC`
 	return fmt.Sprintf(q, tableName)