FredPluginFCPMessageHandler: JavaDoc

Don't merely talk about "synchronous send functions", put a real
JavaDoc link instead.
Notice that there currently is only one such function, so the plural is
dropped.
FredPluginFCPMessageHandler: JavaDoc

Document that the message identifier is an UUID by default, and
recommend people to use UUIDs in alternate implementations as well:
UUIDs are specially crafted to contain enough entropy to be globally
unique each.