test: document default_trait_access suggestion as unfixable (#17569)

changelog: none

The original machine-applicability change is unsafe because
`with_forced_trimmed_paths` can produce a replacement path that is not
in scope. For example, `std::time::Duration` is rendered as `Duration`.

This revision:
- keeps the suggestion at `Applicability::Unspecified`
- documents the path-trimming limitation next to the applicability
- adds a `default_trait_access_unfixable.rs` regression test

## Validation
- `TESTNAME=default_trait_access_unfixable cargo uibless`
- `TESTNAME=default_trait_access_unfixable cargo uitest`
- `cargo fmt --check`