Documentation Needed: Argument definitions/usage for a custom validator

The documentation currently shows that you can create a custom validator and which arguments are provided into the function, however, nowhere is it explained what each of these arguments actually IS. For example, I have a validator that is dependent on two values from the `changeset` and I thought getting the property from the `changes` argument would work but in some cases, I am getting `undefined`. Whether or not this is a bug or just an issue in my code is irrelevant. The problem here is that there is no indication on what these args mean or how they should be used. `key`, `oldValue` and `newValue` are easy enough, but what about `changes` and `content`?