Document "low-trust" events

I discovered that we have this "trust" concept when, for the first time, I saw an error message that referred to it. Hunting around the amphtml repo, I found a few issues and [this file](https://github.com/ampproject/amphtml/blob/master/src/action-constants.js) that refer to it.

So far, I only see the idea referred to obliquely in the [Actions and events page](https://amp.dev/documentation/guides-and-tutorials/learn/amp-actions-and-events/). We should document this!