Missing trust information in the email actions reference

## 📖 Missing or Out-of-Date Documentation

### Problem
The email actions are missing trust descriptions in the event/actions doc. When a low-trust event fires it's unclear what actions are available.

### Location
https://github.com/ampproject/amp.dev/blob/future/pages/content/amp-dev/documentation/guides-and-tutorials/learn/amp-email-actions-and-events.md

### Content type
Reference docs

### Information architecture (optional)
It looks like this used to be documented for [amp-web here](https://github.com/ampproject/amp.dev/pull/2986/files#diff-271c62ac798d620bfa58cc85d34439446c640f67d8d733cbe3662ad194f2ffee).