polish code and fix some documentation errors

[chenlushun]

What is the purpose of the change

polish code and fix some documentation errors

Brief changelog

org.apache.dubbo.registry.nacos.NacosRegistryFactory
org.apache.dubbo.registry.nacos.NacosRegistry

Verifying this change

XXXXX

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[codecov-io]

Codecov Report

Merging #3818 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3818      +/-   ##
============================================
- Coverage     63.64%   63.59%   -0.06%     
  Complexity       71       71              
============================================
  Files           704      704              
  Lines         31065    31065              
  Branches       5037     5037              
============================================
- Hits          19772    19755      -17     
- Misses         9024     9038      +14     
- Partials       2269     2272       +3

Impacted Files	Coverage Δ	Complexity Δ
...ache/dubbo/remoting/p2p/support/AbstractGroup.java	45.45% <0%> (-11.37%)	0% <0%> (ø)
...apache/dubbo/common/config/ConfigurationUtils.java	64% <0%> (-8%)	0% <0%> (ø)
...onfig/spring/extension/SpringExtensionFactory.java	80.48% <0%> (-4.88%)	0% <0%> (ø)
...e/dubbo/remoting/transport/netty/NettyChannel.java	57.64% <0%> (-4.71%)	0% <0%> (ø)
.../exchange/support/header/HeaderExchangeServer.java	66.98% <0%> (-2.84%)	0% <0%> (ø)
.../apache/dubbo/remoting/transport/AbstractPeer.java	65.21% <0%> (-2.18%)	0% <0%> (ø)
...dubbo/remoting/exchange/support/DefaultFuture.java	75.51% <0%> (-0.69%)	0% <0%> (ø)
...pache/dubbo/remoting/transport/AbstractServer.java	47.91% <0%> (+1.04%)	0% <0%> (ø)	⬇️

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[ralf0131]

LGTM, thanks for the pull request.