Document --innobackupex option for MariaDB Backup

The MariaDB Backup documentation page has the following note:

No symlink to innobackupex - use the “--innobackupex” parameter instead.

https://mariadb.com/kb/en/mariadb/mariadb-backup-overview/#limitations

But this option is not documented here:

https://mariadb.com/kb/en/mariadb/mariabackup-options/

We should probably document it. What does the option do? Does it make the "mariabackup" executable option work exactly like the "innobackupex" wrapper from XtraBackup? How does this change its behavior in practical terms?