LUDOC-306 dne2: update with new command for DNE2

LUDOC-306 dne2: update with new command for DNE2

Distributed metadata phase 2 (DNE2) introduces the concept of
a the metadata contents of a given directory striped across
multiple MDTs.

Update the manual to ensure the operation of this new feature is
documented. Include description of migration tool.

See also: LU-3531

Change-Id: I96f7012638358b9985480ec67539cf72adf3ced5
Signed-off-by: Richard Henwood <richard.henwood@intel.com>
Reviewed-on: http://review.whamcloud.com/17394
Tested-by: Jenkins