FredPluginFCPMessageHandler.*.handlePluginFCPMessage(): JavaDoc

The condition at which a reply is allowed was elaborated right in the
previous parapgrah, no need to repeat it. The opposite of returning a
reply (= to return null) was explained more far above in the text, so
repeat that instead.
PrioritizedMessageHandler: JavaDoc

Make it sound less like the message priority *has* to be determined for
individual messages - developers are free to ignore the parameter which
specifies it.