it's possible that on a scan engine only system (no scan controller) that you will not see any logs present until/unless there is either:
1) Scanning occuring
2) Service(s) Restarts (Scan Engine, or Log Services)
The log entries you refer to happening on your MVM3000 could be for other services like:
Notification (if enabled)
It's not necessarily going to be a good measure to monitor the logs on your 2100's. Can you Monitor Services instead?
But to your original question. No there isn't any documentation around it. Quick synopsis for you:
LogToDiskSvc messages are logged whenever the following Services are started, or invoked (present on all MVM Systems):
FSAPI messages are on the MVM System where the API Server is running. That logs Login Attempts and Portal Actions.
FSScanCtrl, ResultProcessor messages are on the MVM System where the ScanController service is running. That logs Scan Start times and ScanEngine<-> Scan Controller communication.
FSNotification are notification events on the Notification Server.
ScanEngineSvc, ScanEngine, Discovery, Assessment, WebScanModule, FASLModule, WebFASLModule, WindowsModule, ShellModule, WirelessModule, are all messages that can appear on the system where the Scan Engine is running.
I hope that helps a bit.
Very verbose answer indeed, great stuff!