Java doc for remote netty4

[wavesZh]

What is the purpose of the change

add some comments for dubbo-remoting-netty4

Brief changelog

XXXXX

Verifying this change

XXXXX

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[wavesZh]

i have merged the latest code into your branch but still check fail.

[wavesZh]

And the problem happen second time at the same UT position

[carryxyh]

Pls format your doc.

[ralf0131]

LGTM.

[codecov-io]

Codecov Report

Merging #4180 into master will increase coverage by <.01%.
The diff coverage is 75%.

@@             Coverage Diff              @@
##             master    #4180      +/-   ##
============================================
+ Coverage     62.87%   62.88%   +<.01%     
- Complexity      504      506       +2     
============================================
  Files           769      769              
  Lines         33058    33058              
  Branches       5217     5217              
============================================
+ Hits          20786    20789       +3     
- Misses         9874     9875       +1     
+ Partials       2398     2394       -4

Impacted Files	Coverage Δ	Complexity Δ
...main/java/org/apache/dubbo/remoting/Constants.java	100% <ø> (ø)	0 <0> (ø)	⬇️
...bo/remoting/transport/netty4/NettyTransporter.java	100% <ø> (ø)	0 <0> (ø)	⬇️
.../remoting/transport/netty4/NettyClientHandler.java	68.42% <ø> (+7.01%)	0 <0> (ø)	⬇️
...e/dubbo/remoting/transport/netty4/NettyServer.java	76.56% <ø> (ø)	0 <0> (ø)	⬇️
...n/java/org/apache/dubbo/common/utils/UrlUtils.java	78.04% <ø> (ø)	0 <0> (ø)	⬇️
...java/org/apache/dubbo/remoting/utils/UrlUtils.java	57.14% <ø> (ø)	0 <0> (ø)	⬇️
.../dubbo/remoting/transport/netty4/NettyChannel.java	65.88% <0%> (ø)	0 <0> (ø)	⬇️
...e/dubbo/remoting/transport/netty4/NettyClient.java	70.58% <100%> (ø)	0 <0> (ø)	⬇️
.../remoting/transport/netty4/NettyServerHandler.java	71.42% <100%> (ø)	0 <0> (ø)	⬇️
.../apache/dubbo/qos/protocol/QosProtocolWrapper.java	64.1% <0%> (-17.95%)	0% <0%> (ø)
... and 10 more

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