Wrong description of slow_query_log_file variable / option

--slow-query-log-file=name
Log slow queries to given log file. Defaults logging to
'hostname'-slow.log. Must be enabled to activate other
slow log options

slow_query_log_file is a string, it cannot be enabled (and it cannot be disabled either, for the option an empty string is ignored and for the variable it is forbidden).

The description is the same for all existing versions.
10.11 is additionally affected as the description has been copied for the new variable log_slow_query_file.

I've set fix versions based on affected versions, but it's okay to fix only in 10.11 if that's preferable on some reason.