LU-1962 doc: add "lfs df --lazy" description

LU-1962 doc: add "lfs df --lazy" description

The "lfs df --lazy" option was previously added in change
http://review.whamcloud.com/4007 but was not documented.
Add a short description to the lfs.1 man page.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I9f29d11ddaadd7b415c655ac43ba2738433ebbe5
Reviewed-on: http://review.whamcloud.com/7689
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Alexander Boyko <alexander_boyko@xyratex.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>