Extend comment to explain why we shouldn't join the thread for the panic example.

[mijoharas]

Hi,
Can we extend this comment here on not joining the thread in the panic handler with an explanation as to why? without joining, I worry that any panic crashing my program will be lost and not submitted to rollbar (which is what happens in the example given).

[mijoharas]

also, to clarify, I guess I'm mainly asking what the reason is? otherwise I'd just raised a PR instead of opening this issue, which I'm happy to do ofc.