docs(sylvia-derive): Document inner types

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.12%. Comparing base (abfa2a5) to head (bbaad91). Report is 2 commits behind head on main.

Files	Patch %	Lines
sylvia-derive/src/entry_points.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #414 +/- ## ========================================== + Coverage 82.11% 82.12% +0.01% ========================================== Files 53 53 Lines 3075 3077 +2 ========================================== + Hits 2525 2527 +2 Misses 550 550 ```

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