Xtrabackup 2.3's documentation is outdated

It’s seems xtrabackup 2.3’s documentation is too old; For example, https://www.percona.com/doc/percona-…_a_backup.html says

Note that xtrabackup backs up only the InnoDB data. You must separately restore the MySQL system database,MyISAM data, table definition files (.frm files), and everything else necessary to make your database functional – orinnobackupex can do it for you.

But I tested, and found that xtrabackup 2.3 could copy non-InnoDB files,so I think documentation is outdated~