[ty] Refactor reStructuredText parameter documentation extraction to use an `IndexMap` from the start. (#25932)
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## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
This helps make the final container type for parameter docs clearer.
Along the way I've also added a few regression tests that lock in the
current precedence of parameter docs across formats.
## Test Plan
Please see included tests.
<!-- How was it tested? -->
ruff
770541e0 - [ty] Refactor reStructuredText parameter documentation extraction to use an `IndexMap` from the start. (#25932)
