MB-10049: revise xdcr-replicate help

Change-Id: I7d979dbfae2997dfa27cb9d491f161834e9c149f
Reviewed-on: http://review.couchbase.org/34590
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>