LU-3531 doc: update lfs migrate and mkdir documentation.

LU-3531 doc: update lfs migrate and mkdir documentation.

lfs mkdir command has it's own man page. Update lfs main man page
to point directly at that page. lfs migrate is updated to accurately
record usage and limitations.

Signed-off-by: Richard Henwood <richard.henwood@intel.com>
Change-Id: Ie247f1f267a4d55a43ce5aaa734c8b8da487771c
Reviewed-on: http://review.whamcloud.com/17392
Reviewed-by: wangdi <di.wang@intel.com>
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>