opt: improve comments on Conn and test cases

[panjf2000]

No description provided.

[codecov]

Codecov Report

Patch coverage: 60.00% and project coverage change: -0.08 ⚠️

Comparison is base (b341103) 77.04% compared to head (9af57b2) 76.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #471      +/-   ##
==========================================
- Coverage   77.04%   76.97%   -0.08%     
==========================================
  Files          23       23              
  Lines        2418     2445      +27     
==========================================
+ Hits         1863     1882      +19     
- Misses        423      429       +6     
- Partials      132      134       +2     

Flag	Coverage Δ
unittests	76.97% <60.00%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
connection_unix.go	71.20% <0.00%> (ø)
connection_windows.go	64.52% <0.00%> (ø)
gnet.go	86.02% <ø> (+2.20%)	⬆️
reactor_default_bsd.go	87.14% <ø> (-0.19%)	⬇️
reactor_default_linux.go	82.02% <ø> (-0.20%)	⬇️
reactor_optimized_bsd.go	80.85% <ø> (-0.40%)	⬇️
reactor_optimized_linux.go	80.85% <ø> (-0.40%)	⬇️
listener_windows.go	78.57% <63.63%> (-9.36%)	⬇️
client_unix.go	58.06% <100.00%> (+0.27%)	⬆️
client_windows.go	79.59% <100.00%> (+0.13%)	⬆️
... and 2 more

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