FredPluginFCPMessageHandler: Make the JavaDoc flow of thought more
well-connected

- Improve the wording of the section which tells server authors to not
keep a hard reference of the FCPPluginClient to make it more obvious
that the section below tells them how they can reference a client by
its UUID instead of the Java hard reference
- Also make the section below use the phrase "keep a reference" so it is
obvious that it explains the above