FredPluginFCPMessageHandler...handlePluginFCPMessage(): Document that
people should always return a reply

This is identical JavaDoc for the same function at both interfaces which
contain it. (No JavaDoc is at the parent interface because we
explicitely tell people to study the *different* primary JavaDoc of that
function in the two interfaces.)

Also explain why:
- To allow the remote side to detect errors easily
- To allow sendSynchronous() to fail fast.