Rollup merge of #135406 - Aditya-PS-05:fix/unstable-lint-docs, r=compiler-errors

Update unstable lint docs to include required feature attributes

closes #135298

## Summary
This PR updates the documentation examples for the following unstable lints to ensure they include the necessary feature attributes for proper usage:

- fuzzy_provenance_casts
- lossy_provenance_casts
- unqualified_local_imports
- test_unstable_lint

## Changes Made:

- Added the appropriate #![feature(...)] attributes to the example code for each lint.
- Updated the examples to produce correct and meaningful warnings, ensuring they align with current lint behavior.

Reference:
- Used the `must_not_suspend` lint documentation as a template for these updates.

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