### What changes were proposed in this pull request?
Ratis-shell use GRPC by default. Celeborn support Netty for ratis, if `raft.rpc.type` is not specified, commands may fail.
e.g.
```
org.apache.ratis.thirdparty.io.grpc.StatusRuntimeException: DEADLINE_EXCEEDED: deadline exceeded after 14.947369960s. [closed=[], open=[[buffered_nanos=14962358255, waiting_for_connection]]]
```
So I think we should update the document to mention how to change the RPC type to in `celeborn-ratis`.
### Why are the changes needed?
Improve user experience
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
Manually test
Closes#1530 from onebox-li/ratis-shell-default-rpc.
Lead-authored-by: liyihe <liyihe@bigo.sg>
Co-authored-by: Cheng Pan <pan3793@gmail.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>
Licensed to the Apache Software Foundation (ASF) under one or more
contributor license agreements. See the NOTICE file distributed with
this work for additional information regarding copyright ownership.
The ASF licenses this file to You under the Apache License, Version 2.0
(the "License"); you may not use this file except in compliance with
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http://www.apache.org/licenses/LICENSE-2.0
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See the License for the specific language governing permissions and
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