Rollup merge of #144185 - purplesyringa:poisoning-wording, r=Amanieu

Document guarantees of poisoning

This mostly documents the current behavior of `Mutex` and `RwLock` (rust-lang/rust#143471) as imperfect. It's unlikely that the situation improves significantly in the future, and even if it does, the rules will probably be more complicated than "poisoning is completely reliable", so this is a conservative guarantee.

We also explicitly specify that `OnceLock` never poisons, even though it has an API similar to mutexes.

Fixes rust-lang/rust#143471 by improving documentation.

r? ``@Amanieu``

Author: samueltardieu