sync::Mutex: Add note about the absence of poisoning (#1933)

tokio/src/sync/mutex.rs

@@ -26,6 +26,11 @@
 //! }
 //! ```
 //!
+//! Note that in contrast to `std::sync::Mutex`, this implementation does not
+//! poison the mutex when a thread holding the `MutexGuard` panics. In such a
+//! case, the mutex will be unlocked. If the panic is caught, this might leave
+//! the data protected by the mutes in an inconsistent state.
+//!
 //! [`Mutex`]: struct.Mutex.html
 //! [`MutexGuard`]: struct.MutexGuard.html