LU-8065 utils: clarify lctl help messages

LU-8065 utils: clarify lctl help messages

Clarify the syntax help messages of lctl changelog_{de}register
and make them consistent with the other ones.

Test-Parameters: trivial

Change-Id: Iffb36906d764b5832722e635ab6b9feadf38c64c
Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-on: http://review.whamcloud.com/19764
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>