Event Notifications - modifying email text

I'd like to tweak the email content when an Event Trigger is activated. Here's what I have now:

Device: ${eventSummary/actor/element_title}
Component: ${evt/component}
Severity: ${evt/severity}
Time: ${evt/lastTime}
Message:
${evt/message}

To start with, I'd like to modify the Severity section. At the moment ${evt/severity} brings back a number between 1 and 6. I'd prefer it to show a descriptive word instead, i.e. INFO, WARN, ERR, etc. Is there a $ parameter that I could use for that.

With that in mind, is there a definitive list of all these $ parameters anyway?

Thanks