fix: Add comments for re-writing properties from Koa

[ghost]

Because the version of Koa is updated to the latest version,
we cannot simply remove properties but just re-write properties
from the inherited ones. Add more comments on this.

Ref：#3329

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[codecov]

Codecov Report

Merging #3332 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3332   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files          29       29           
  Lines         844      844           
=======================================
  Hits          842      842           
  Misses          2        2

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