FCPPluginClient.sendSynchronous(): Remove self-explanatory JavaDoc
FCPPluginClient.send(): JavaDoc parameter message

- This is inspired by the same parameter of sendSynchronous().
- It shall stress that identifier collision must be prevented to not
confuse sendSynchronous(). But not to the same level: Only reply
messages can confuse sendSynchronous(), and those are typically not sent
with send() by the client/server plugin. It will instead just return the
reply in the message handler, and the internal fred code will do send().