Mod: add comments for remoting

[watermelo]

What this PR does:
add comments for remoting

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #605 into develop will increase coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #605      +/-   ##
===========================================
+ Coverage    67.21%   67.37%   +0.16%     
===========================================
  Files          188      190       +2     
  Lines         9852     9917      +65     
===========================================
+ Hits          6622     6682      +60     
  Misses        2580     2580              
- Partials       650      655       +5     

Impacted Files	Coverage Δ
registry/etcdv3/registry.go	73.13% <ø> (ø)
remoting/etcdv3/client.go	57.80% <ø> (ø)
remoting/etcdv3/facade.go	0.00% <ø> (ø)
remoting/etcdv3/listener.go	46.49% <ø> (+0.40%)	⬆️
remoting/kubernetes/client.go	76.25% <ø> (ø)
remoting/kubernetes/listener.go	52.83% <ø> (ø)
remoting/kubernetes/registry_controller.go	55.67% <ø> (ø)
remoting/kubernetes/watch.go	78.89% <ø> (ø)
remoting/zookeeper/client.go	68.68% <ø> (ø)
remoting/zookeeper/facade.go	70.27% <ø> (ø)
... and 12 more

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[fangyincheng]

LGTM