docs: add more in-code docs to hudi-core APIs

[xushiyan]

Description

Update in-code docs and tests for public APIs in hudi-core.

Closes #71

How are the changes test-covered

• [ ] N/A
• [x] Automated tests (unit and/or integration tests)
• [ ] Manual tests
  • [ ] Details are described below

[codecov[bot]]

Codecov Report

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

Project coverage is 89.81%. Comparing base (d5f42f6) to head (da13196). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #166 +/- ## ======================================= Coverage 89.81% 89.81% ======================================= Files 17 17 Lines 884 884 ======================================= Hits 794 794 Misses 90 90 ```

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