doc about system variables say "max purge thread can only set 1" ,but it dose not

in https://mariadb.com/kb/en/mariadb/documentation/storage-engines/xtradb-and-innodb/xtradbinnodb-server-system-variables/#innodb_purge_threads

innodb_purge_threads

Description: Number of threads dedicated to XtraDB purge operations. If set to 0, the default, purging is done with the master thread. If set to 1, the current maximum, purging is done on a separate thread with could reduce contention. Currently has minimal effect, but is a future area of improvement. See also innodb_purge_batch_size.
Commandline: --innodb-purge-threads=#
Scope: Global
Dynamic: No
Data Type: numeric
Default Value: 0
Range: 0 to 1
Introduced: MariaDB/MySQL 5.5.4

but i write "innodb-purge-threads=2" in /etc/my.cnf ,and restart server,and run "show variables like 'innodb-purge-threads'" ,it return 2 ,but document says "Range: 0 to 1"