There are two separate mechanisms in play.
1. Find My iPhone. This is a mechanism that allows the account holder to locate other devices tied to that account. For example, a parent could put childrens' phones on their account to keep track of the phone locations. You set up Find My iPhone by setting a specific switch in the device's iCloud settings.
2. iCloud Keychain. This is where the term "trusted" comes in. Trusted devices mutually share login credentials and other auto-fill information. You set up the iCloud Keychain via a separate switch in the device's iCloud settings.
In other words, I might want to find a phone but not trust it with my bank passcodes. So when the iCloud Keychain instructions say that there is a list of trusted devices, the instructions are wrong. There is no way to tell if devices are trusted or not, only if they are findable or not.
I've been looking at other threads on this subject, and apparently there USED to be a list of trusted devices that you reached through "Manage My Apple ID." At least, I found a year-old discussion post describing the list.