LU-930 doc: update LFSCK documentation

LU-930 doc: update LFSCK documentation

Update the LFSCK usage document to include information about logging
LFSCK changes to the console or separate log file.

Improve the description of some options, putting more commonly-used
options first.

Update URLs to reference lustre.org instead of other websites.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I980a43143297400c0a544281b716c32881d65172
Reviewed-on: https://review.whamcloud.com/26253
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Joseph Gmitter <joseph.gmitter@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>