Update user definitions page with technical info on how we count users

Created by: attfarhan

Provide some context as to how users are actually calculated in the app. This question comes up often, and has caused some confusion, so it makes sense to clearly document this.

I want to dig deeper into this and will add more as I find more, but this is a good start and is useful as is. @ebrodymoore can you double check what I've written here?