add message about @objc_block

Author: nuclearace

SwiftIO/SocketAckMap.swift

@@ -24,6 +24,10 @@
 
 import Foundation
 
+// @objc_block is undocumented, but is used because Swift assumes that all
+// Objective-C blocks are copied, but Objective-C assumes that Swift will copy it.
+// And the way things are done here, the bridging fails to copy the block in
+// SocketAckMap#addAck
 public typealias AckCallback = @objc_block (NSArray?) -> Void
 public typealias OnAckCallback = (timeout:UInt64, callback:AckCallback) -> Void