Hey Luke,
This process changed with the spring release update for two way relationships (see post).
So you should be using Entity Relationship set to One Way relationship type. The validation should be “FAQs” as you mentioned in the first option. That is what is meant by “limited to just FAQs".
I would just make sure that you are adding this custom field to the Jobs entity type under job details as well as that could be causing the rule not to pass.
Let me know if this works for you. I will also update the instructions and make sure the wording is clear. Thank you for catching those issues!