Improve documentations

[fzyzcjy]

Changes

Please list issues fixed by this PR here, using format "Fixes #the-issue-number".

Checklist

• [ ] An issue to be fixed by this PR is listed above.
• [ ] New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• [ ] ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• [ ] If this PR adds/changes features, documentations (in the ./website folder) are updated.
• [ ] CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.79%. Comparing base (8915d06) to head (84f5b81).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1919 +/- ## ========================================== - Coverage 99.25% 97.79% -1.47% ========================================== Files 365 365 Lines 15533 15497 -36 ========================================== - Hits 15417 15155 -262 - Misses 116 342 +226 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.