Restore full API in docstrings

[gdalle]

Checklist

• [x] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

Additional context

Fix #56 by reinstating full docstrings with type parameters and fields

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (3d4e912) to head (0921565).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #57 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 6 6 Lines 65 65 ========================================= Hits 65 65 ``` | [Flag](https://app.codecov.io/gh/SciML/ADTypes.jl/pull/57/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciML) | Coverage Δ | | |---|---|---| | [docs](https://app.codecov.io/gh/SciML/ADTypes.jl/pull/57/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciML) | `2.56% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SciML#carryforward-flags-in-the-pull-request-comment) to find out more.

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