Tracking issue for release notes of #128034: exhaustiveness: Explain why a given pattern is considered unreachable

[rustbot]

This issue tracks the release notes text for #128034.

• Issue is nominated for the responsible team (and T-release nomination is removed).
• Proposed text is drafted by team responsible for underlying change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text:

The section title will be de-duplicated by the release team with other release notes issues.
Prefer to use the standard titles from previous releases.
More than one section can be included if needed.

# Compiler
- [Improved diagnostics to explain why a pattern is unreachable](https://github.com/rust-lang/rust/pull/128034)

Release blog section (if any, leave blank if no section is expected):

[wesleywiser]

T-compiler updated the release notes text during the weekly compiler team triage meeting.

[jieyouxu]

Also a remark:

We could maybe have a diagnostic improvement section if we want to highlight efforts trying to continuously improve diagnostics and emphasize the importance of good diagnostics.