Document how ptr::drop_in_place behaves when dropping a slice panics

[gnzlbg]

When dropping a slice [T] using ptr::drop_in_place, a T::drop can panic. I can't find any documentation of what the behavior of the program is in that case.

[gnzlbg]

I've posted on the open issue in the reference (rust-lang/reference#348 (comment)). Once that is resolved, we should update the docs here:

[hkBst]

I've posted on the open issue in the reference (rust-lang/reference#348 (comment)). Once that is resolved, we should update the docs here:

This is still open, so:

@rustbot blocked

[rustbot]

Error: The "Blocked" shortcut only works on pull requests.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.