LUDOC-11 misc: correct location/setting qos_threshold_rr

LUDOC-11 misc: correct location/setting qos_threshold_rr

The parameter qos_threshold_rr has moved from /sys/fs/.../lov
to lod and, to set this parameter, the command 'lctl set_param'
command should be run on the MDS.  These two things need to be
corrected in the manual.

Change-Id: Ide6d809d2d36e36504d014fb511df82ada80d977
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36695
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>