Document the disconnection mechanism of clients towards
which were passed to FredPluginFCPMessageHandler server plugins

- Servers who store client UUIDs shall periodically:
* Obtain the FCPPluginClient by PluginRespirator.getPluginClientByID()
* Try to send a message to it with FCPPluginClient.send()
* Discard the client UUID if any of the two functions throws
IOExceptions
- This is necessary to prevent excessive growth of the client UUID table
at server plugins.
- It is documented the most at the
FredPluginFCPMessageHandler.ServerSideMessageHandler because that is the
place where the server gets the FCPPluginClient object from and where it
is instructed by the previous JavaDoc already that it may store the
UUID. Additional JavaDoc is provided at the general
overview of plugin FCP at FCPPluginClient, and at
PluginRespirator.getPluginClientByID() to ensure that users of the
function do not forget about this