Document interaction with destructors

[Techcable]

The interaction with destructors and #[nounwind] was discussed in rust-lang/rust#129582. The reference was changed in rust-lang/reference#1226 to state the following:

With panic=unwind, when a panic is turned into an abort by a non-unwinding ABI boundary, either no destructors (Drop calls) will run, or all destructors up until the ABI boundary will run. It is unspecified which of those two behaviors will happen.

The latter case is problematic and needs to be documented.