document the simplicity of fetch

layeredcache.go

@@ -131,6 +131,9 @@ func (c *LayeredCache) Replace(primary, secondary string, value interface{}) boo
 // Attempts to get the value from the cache and calles fetch on a miss.
 // If fetch returns an error, no value is cached and the error is returned back
 // to the caller.
+// Note that Fetch merely calls the public Get and Set functions. If you want
+// a different Fetch behavior, such as thundering herd protection or returning
+// expired items, implement it in your application.
 func (c *LayeredCache) Fetch(primary, secondary string, duration time.Duration, fetch func() (interface{}, error)) (*Item, error) {
 	item := c.Get(primary, secondary)
 	if item != nil {