Clarify that the OnEvicted function isn't called when an item is overwritten

cache.go

@@ -853,8 +853,8 @@ func (c *cache) DeleteExpired() {
 }
 
 // Sets an (optional) function that is called with the key and value when an
-// item is evicted from the cache. (Including when it is deleted manually.)
-// Set to nil to disable.
+// item is evicted from the cache. (Including when it is deleted manually, but
+// not when it is overwritten.) Set to nil to disable.
 func (c *cache) OnEvicted(f func(string, interface{})) {
 	c.mu.Lock()
 	defer c.mu.Unlock()