Add safety comments

Author: szymon-zadworny

oneapi-rs/src/event.rs

@@ -57,6 +57,8 @@ impl Future for EventFuture {
             *this.set_callback = true;
             let mut queue = Queue::new_immediate();
             this.shared.waker.register(cx.waker());
+            // Safety: the SharedWaker will always outlive the C++ host task.
+            // Safety: the Future which holds the SharedWaker is pinned - the pointer will remain valid.
             unsafe { ffi::register_callback(&mut queue.0, &this.event.0, this.shared) };
             this.queue.replace(queue);
         }
@@ -66,7 +68,6 @@ impl Future for EventFuture {
                 return Poll::Ready(());
             }
 
-            // Register the waker if result isn't ready. This is a slow atomic operation
             this.shared.waker.register(cx.waker());
         }