Document that RwLock might panic

[Like a `Mutex`](https://doc.rust-lang.org/std/sync/struct.Mutex.html#panics), [`RwLock` might panic](https://github.com/rust-lang/rust/blob/f796112/src/libstd/sys/unix/rwlock.rs#L59) if the current thread already holds a lock. This however is [not documented](https://doc.rust-lang.org/std/sync/struct.RwLock.html).
