ServerSideMessageHandler: JavaDoc

Fix wrong claims about client disconnection: IOException is not
necessarily thrown if a client has disconnected. Thus fix the JavaDoc to
be more similar to the one at PluginRespirator.connectToOtherPlugin().
You can also see FCPPluginClient.send() for details.
FredPluginFCPMessageHandler.*.handlePluginFCPMessage(): JavaDoc

Explain in more detail why the handler should return the reply instead
of calling FCPPluginClient.send().
FredPluginFCPMessageHandler.*.handlePluginFCPMessage(): JavaDoc

Remove JavaDoc which says that the handler must not throw
RuntimeException: While this is of course not recommended behavior,
FCPPluginClient.send() will catch and log RuntimeExceptions anyway. So
we're safe. And I doubt that any sane developer would randomly throw
stuff out to the node.