Mutex and RwLock need RefUnwindSafe too
Incomplete, because I don't know what the appropriate stability annotation is here, but this is an attempt to bring the documentation for `std::panic` in line with reality. Right now, it says:
>Types like `&Mutex<T>`, however, are unwind safe because they implement poisoning by default.
But only `Mutex<T>`, not `&Mutex<T>`, is unwind-safe.
617039bff0