FredPluginFCPMessageHandler: Fix JavaDoc links to old API
- The PluginTalker was the class which was used to send messages
- The PluginReplySender was the class which was used to send replies
- The FredPluginFCP was the message handler interface which received the messages
- The FredPluginTalker was the message handler interface which received the replies.
FredPluginFCPMessageHandler: Improve JavaDoc which describes the difference to the old API