branches/5.1: ha_innobase::delete_all_rows(): In response to a user asking

<http://forums.innodb.com/read.php?4,215,215> why DELETE FROM t is not
mapped to TRUNCATE TABLE t as it is in MyISAM, note in a comment that
DELETE is transactional while TRUNCATE is not.

handler/ha_innodb.cc

@@ -5335,7 +5335,8 @@ ha_innobase::delete_all_rows(void)
 	if (thd_sql_command(user_thd) != SQLCOM_TRUNCATE) {
 	fallback:
 		/* We only handle TRUNCATE TABLE t as a special case.
-		DELETE FROM t will have to use ha_innobase::delete_row(). */
+		DELETE FROM t will have to use ha_innobase::delete_row(),
+		because DELETE is transactional while TRUNCATE is not. */
 		DBUG_RETURN(my_errno=HA_ERR_WRONG_COMMAND);
 	}