LU-930 doc: update lfs migrate usage and man page

LU-930 doc: update lfs migrate usage and man page

Update the usage and man page for "lfs migrate -m", noting that
this command will recursively migrate an entire directory tree.

It is not currently possible to migrate files with DoM components
between MDTs, so provide an example of how to work around this.

Only print the command-line options for commands in the usage
message instead of the full usage, since it is otherwise much
too verbose to see the actual error message being printed.  The
user should read the lfs-migrate.1 man page for full usage.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I9e34a33fcc3f0e2b90bc499fb7b946c53e6111d1
Reviewed-on: https://review.whamcloud.com/43378
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>