Add documentation for antireplay package

antireplay/noop.go

@@ -6,6 +6,9 @@ type noop struct{}
 
 func (n noop) SeenBefore(_ []byte) bool { return false }
 
+// NewNoop returns an implementation that does nothing. A corresponding
+// method always returns false, so this cache accepts everything you
+// pass to it.
 func NewNoop() mtglib.AntiReplayCache {
 	return noop{}
 }