add xds futrue note

[wucheng1997]

What is the purpose of the change

这一块代码的future是在StreamObserver onNext的方法里面进行complete，不联系上下文很难理解，以为是future的新特性。
身边几个人阅读时都无法理解这段代码，可读性偏差。增加注释增加可读性。

Brief changelog

Verifying this change

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Check if is necessary to patch to Dubbo 3 if you are work on Dubbo 2.7
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Add some description to dubbo-website project if you are requesting to add a feature.
• GitHub Actions works fine on your own branch.
• If this contribution is large, please follow the Software Donation Guide.

[codecov-commenter]

Codecov Report

Merging #10760 (85f5545) into 3.2 (c657546) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.2   #10760      +/-   ##
============================================
- Coverage     65.33%   65.33%   -0.01%     
- Complexity      392      393       +1     
============================================
  Files          1341     1341              
  Lines         57180    57180              
  Branches       8451     8451              
============================================
- Hits          37361    37356       -5     
- Misses        15861    15866       +5     
  Partials       3958     3958              

Impacted Files	Coverage Δ
...o/registry/xds/util/protocol/AbstractProtocol.java	0.00% <ø> (ø)
...rg/apache/dubbo/rpc/protocol/dubbo/DubboCodec.java	65.21% <0.00%> (-4.35%)	⬇️
...dubbo/remoting/exchange/support/DefaultFuture.java	87.93% <0.00%> (-4.32%)	⬇️
...bo/rpc/protocol/dubbo/DecodeableRpcInvocation.java	77.88% <0.00%> (-3.85%)	⬇️
...ache/dubbo/rpc/protocol/dubbo/DubboCountCodec.java	90.62% <0.00%> (-3.13%)	⬇️
.../apache/dubbo/remoting/transport/CodecSupport.java	81.53% <0.00%> (-3.08%)	⬇️
.../dubbo/registry/client/metadata/MetadataUtils.java	69.44% <0.00%> (-2.78%)	⬇️
.../dubbo/rpc/protocol/dubbo/DecodeableRpcResult.java	60.25% <0.00%> (-2.57%)	⬇️
...org/apache/dubbo/rpc/protocol/AbstractInvoker.java	72.27% <0.00%> (-1.99%)	⬇️
...adata/store/zookeeper/ZookeeperMetadataReport.java	72.34% <0.00%> (-1.07%)	⬇️
... and 14 more

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