update unimplemented! docs

[MarkMcCaskey]

For #42628 (updating docs from changes from #42155).

Initial changes made to make unimplemented! doc comments look more like unreachable! and remove statement about the panic message.

r? @steveklabnik

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[bors]

☔ The latest upstream changes (presumably #44186) made this pull request unmergeable. Please resolve the merge conflicts.

[aidanhs]

Thanks for the PR @MarkMcCaskey! We'll check in now and again to make sure @steveklabnik or another reviewer gets to this soon.

[aidanhs]

(in the meantime, I note there are merge conflicts - it'd be great if you could resolve those)

[MarkMcCaskey]

Okay! Sorry about the long delay. Should be fine now.

[steveklabnik]

@bors: r+ rollup

thank you!

[bors]

📌 Commit 570ae39 has been approved by steveklabnik