docs: clarify XInfoGroup.Lag field behavior with Nil values

Author: ofekshenawa

command.go

@@ -2187,6 +2187,7 @@ func (cmd *XInfoGroupsCmd) readReply(rd *proto.Reader) error {
 
 				// lag: the number of entries in the stream that are still waiting to be delivered
 				// to the group's consumers, or a NULL(Nil) when that number can't be determined.
+				// In that case, we return -1.
 				if err != nil && err != Nil {
 					return err
 				} else if err == Nil {