add more docs

[loongs-zhang]

// Describe your PR here; eg. Fixes #issueNo

Make sure that:

• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.74%. Comparing base (bcfb4b7) to head (4395e7f).

Files with missing lines	Patch %	Lines
core/src/syscall/unix/setsockopt.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
- Coverage   74.81%   74.74%   -0.07%     
==========================================
  Files          72       72              
  Lines        5431     5409      -22     
==========================================
- Hits         4063     4043      -20     
+ Misses       1368     1366       -2     

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