LUDOC-62 admin: better OST/MDT migrate description

LUDOC-62 admin: better OST/MDT migrate description

Instead of restating the backup/restore steps for OST/MDT migration,
reference the existing file/device backup/restore process.  Clean up
the formatting of backup/restore examples a bit.

Update the OST object-to-MDS file mapping process to reference new
MDT FIDs and fid2path, in preference to the old 1.8 debugfs method.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ida33c2f055cf9f66fa1ec7463e1ca36875aecab0
Reviewed-on: http://review.whamcloud.com/3827
Tested-by: Hudson
Reviewed-by: Richard Henwood <Richard.Henwood@intel.com>
Reviewed-by: Cliff White <cliffw@whamcloud.com>