message: doc: add prefetch bandwidth ratelimit example
diff: @@ -123,7 +123,9 @@ We are working on enabling cloud-hypervisor support for nydus.
  // Prefetch thread count
  "threads_count": 10,
  // Maximal read size per prefetch request, e.g. 128kb
  - "merging_size": 131072
  + "merging_size": 131072,
  + // Limit prefetch bandwidth to 1MB/S, it aims at reducing congestion with normal user io
  + "bandwidth_rate": 1048576
  }
  }
  ```
  