Use Display in top-level example for PanicInfo

[lukas-code]

Addresses #110098.

This confused me as well, when I was writing a no_std panic handler for the first time, so here's a better top-level example.

Display is stable, prints the .message() if available, and falls back to .payload().downcast_ref<&str>() if the message is not available. So this example should provide strictly more information and also work for formatted panics.

The old example still exists on the payload method.

[rustbot]

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[JohnTitor]

Great, thank you!
r? @JohnTitor @bors r+ rollup

[bors]

📌 Commit d9f99c3 has been approved by JohnTitor

It is now in the queue for this repository.