Explain iff in mutex.md (#810)

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Co-authored-by: Martin Geisler <martin@geisler.net>

src/concurrency/shared_state/mutex.md

@@ -32,7 +32,7 @@ implementation.
     * It is not possible to forget to acquire the mutex before accessing the protected data.
 * You can get an `&mut T` from an `&Mutex<T>` by taking the lock. The `MutexGuard` ensures that the
   `&mut T` doesn't outlive the lock being held.
-* `Mutex<T>` implements both `Send` and `Sync` iff `T` implements `Send`.
+* `Mutex<T>` implements both `Send` and `Sync` iff (if and only if) `T` implements `Send`.
 * A read-write lock counterpart - `RwLock`.
 * Why does `lock()` return a `Result`? 
     * If the thread that held the `Mutex` panicked, the `Mutex` becomes "poisoned" to signal that