LU-9303 doc: man pages for snapshot components

LU-9303 doc: man pages for snapshot components

Including the separated lctl-barrier and lctl-snapshot man
pages, the lctl-fork-lcfg and lctl-erase-lcfg sections in
the lctl man page. Also some other small cleanup.

Test-parameters: trivial

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ied60b824afcfab5d2d030f12d39c9aa1bab0969a
Reviewed-on: https://review.whamcloud.com/26453
Tested-by: Jenkins
Reviewed-by: Joseph Gmitter <joseph.gmitter@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>