LU-6051 lfs: Update lfs_migrate man page for in-use files

LU-6051 lfs: Update lfs_migrate man page for in-use files

Update man page to state that it is safe to use the script on in-use
files for versions at or above 2.5.

Test-Parameters: trivial
Signed-off-by: Steve Guminski <stephenx.guminski@intel.com>
Change-Id: I412ee7681db3860ca395c4afc2a30c87f1f49d6d
Reviewed-on: https://review.whamcloud.com/29950
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>