Text for slave_connections_needed_for_purge refers to non-existent variable

select variable_comment from information_schema.system_variables where variable_name like 'slave_conne%'\G

*************************** 1. row ***************************

variable_comment: Minimum number of connected slaves required for automatic binary log purge with 

  max_binlog_total_size, binlog_expire_logs_seconds or binlog_expire_logs_days.