Update notes

cache.go

@@ -74,7 +74,7 @@ func (c *cache) Set(k string, x interface{}, d time.Duration) {
 	// adds ~200 ns (as of go1.)
 	c.mu.Unlock()
 
-	// try to call onEvicted if key existed before but the item.Object was replaced
+	// try to call onEvicted if key existed before but it was expired before cleanup
 	if evicted && item.Expired() {
 		c.onEvicted(k, item.Object)
 	}