Unfortunately I don't know of any documentation or comprehensive list. It would be useful to have one, perhap repost this request in the ideas for storage area.
I don't have any of them documented anywhere, but the alerts that are generated for e-mail mostly come from the status logs (pop-ups that you'd see in the UI). They're all quite simplistic in wording.
The e-mail alert subjects are generally ReadyNAS Alert Message ({HostName} {Category}: {Severity})]].
Some alert style messages are:
Failed to sync ADS account information for the realm 'domain.com'
System is shutting down.
Volume usage exeeds....
You can also get backup job logs in e-mail.
It'll also alert you of drive failure telling you which bays and relative chassis.
