docs: Document struct raft_event and struct raft_message

[codecov[bot]]

Codecov Report

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

Comparison is base (38855cc) 74.14% compared to head (02fd566) 74.09%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #176 +/- ## ========================================== - Coverage 74.14% 74.09% -0.05% ========================================== Files 52 52 Lines 10360 10360 Branches 2466 2466 ========================================== - Hits 7681 7676 -5 - Misses 1323 1329 +6 + Partials 1356 1355 -1 ```

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