doc: add documentation for Queue.Done

fetch.go

@@ -156,6 +156,8 @@ func (q *Queue) Block() {
 	<-q.done
 }
 
+// Done returns a channel that is closed when the Queue is closed (either
+// via Close or Cancel). Multiple calls always return the same channel.
 func (q *Queue) Done() <-chan struct{} {
 	return q.done
 }