Clarify what NoExpiration and DefaultExpiration are

cache.go

@@ -24,7 +24,11 @@ func (item *Item) Expired() bool {
 }
 
 const (
+	// For use with functions that take an expiration time.
 	NoExpiration      time.Duration = -1
+	// For use with functions that take an expiration time. Equivalent to
+	// passing in the same expiration duration as was given to New() or
+	// NewFrom() when the cache was created (e.g. 5 minutes.)
 	DefaultExpiration time.Duration = 0
 )