Document panic-in-panic ("double panic") behavior

[ehuss]

I can't find any documentation about double panic behavior (other than https://doc.rust-lang.org/std/ops/trait.Drop.html#panics, which currently just says "will likely abort the program"). There are some subtleties about this, because some handlers will cause a hang (see rust-lang/rust#97146).

I'm not sure exactly where or how to document this. Some thoughts:

• Would be good to add to the new panic chapter in panic runtime and C-unwind documentation #1226
• Might be good to include a note for panic_handler attribute
• See also Document guarantees around drop and panicing #348 which is essentially the same thing. It might be good to include a note in https://doc.rust-lang.org/nightly/reference/destructors.html for that one.

[chorman0773]

Note that the behaviour of the default (in rustc libstd) panic handler is interesting here, and I don't know how much of that is stable or implementation detail:

• On the first panic, it runs the panic hook, then executes the default action depending on the runtime - unwind or abort,
• On the second panic, it enters the panic hook as well (which can cause it to reenter if it was the panic hook, rather than a destructor, that panicked for a second time), then aborts immediately.
• If a panic happens for the third time, it just instantly aborts the process without running the panic hook.