document the simplicity of fetch

cache.go

@@ -136,6 +136,9 @@ func (c *Cache) Replace(key string, value interface{}) bool {
 // Attempts to get the value from the cache and calles fetch on a miss (missing
 // or stale item). 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 *Cache) Fetch(key string, duration time.Duration, fetch func() (interface{}, error)) (*Item, error) {
 	item := c.Get(key)
 	if item != nil && !item.Expired() {

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 {

readme.md

@@ -86,6 +86,8 @@ item, err := cache.Fetch("user:4", time.Minute * 10, func() (interface{}, error)
 })
 ```
 
+`Fetch` doesn't do anything fancy: it merely uses the public `Get` and `Set` functions. If you want more advanced behavior, such as using a singleflight to protect against thundering herd, support a callback that accepts the key, or returning expired items, you should implement that in your application. 
+
 ### Delete
 `Delete` expects the key to delete. It's ok to call `Delete` on a non-existent key: