I am trying to define the standard Oracle responsibilities so that people will know which responsibility to request so that they can perform x. Is their a user guide that defines responsibilities, such as AP Manager can do x, AP specialist can perform these basic functions.
Not really - you can print off what functions are on a menu attached to a particular responsibility (which is a little more complex if you use RBAC), however that is a bit meaningless because there will be loads of functions on a menu and the descriptions of them aren't massively meaningless. Plus you'll need to make considerations for function/menu exclusions, entries without prompts etc.
Ultimately, there is nothing that says "this responsibility can raise invoices" or similar. Unfortunately.