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

All modified and coverable lines are covered by tests ✅

Project coverage is 74.84%. Comparing base (051296f) to head (f453dbc).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #378   +/-   ##
=======================================
  Coverage   74.84%   74.84%           
=======================================
  Files          72       72           
  Lines        5415     5415           
=======================================
  Hits         4053     4053           
  Misses       1362     1362           

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