rgw: admin: Add --shard-id option to help

http://tracker.ceph.com/issues/5324 fixes #5324
Explains operations dependency,
which is more important than the semantic.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
rgw-admin: Adds option --shard-id to help.t

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #382 from kri5/wip-5324

rgw: admin: Add --shard-id option to help

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>