Add introduction for `@as_record` syntax

[codecov[bot]]

Codecov Report

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

Comparison is base (a6891f8) 61.18% compared to head (46785b0) 61.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #180 +/- ## ========================================== + Coverage 61.18% 61.52% +0.33% ========================================== Files 28 28 Lines 1636 1627 -9 ========================================== Hits 1001 1001 + Misses 635 626 -9 ```

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