I learned both Ruby and RoR simultaneously on the job and somehow managed to avoid the pitfall of confusing the two. Maybe it's my interest in languages that makes me look at the note on the doc page that indicates it part of ActiveSomethingOrOther vs Ruby since version x.

I don't know if it will matter for you, but I've found it useful to know what it is that I know. If this seems like you then pay attention to those notes whenever you see them mentioned. I don't specifically look for it but if mentioned on a doc page or StackOverflow comment will pick up on it.