LU-930 man: better describe conf_parm -d in lctl.8

LU-930 man: better describe conf_parm -d in lctl.8

Add a bit more description to "lctl conf_param -d".

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ib7a71122ea3e77e910d817a0ecdc4e6d1ddfd2eb
Reviewed-on: https://review.whamcloud.com/30241
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>