demonstrate the benefit of the mutex

exercises/091_async7.zig

@@ -52,6 +52,12 @@ fn increment(io: std.Io, state: *SharedState, times: u32) void {
         state.mutex.??? catch return;
         defer state.mutex.unlock(); // <-- what's missing here?
 
+        // Sleep to give the other tasks a chance to run in the meantime.
+        // We do this here only to make nondeterminism more visible.
+        io.sleep(std.Io.Duration.fromMilliseconds(1), .awake) catch {};
+
+        // What happens if you neglect to lock the mutex?
+
         state.counter += 1;
     }
 }