Link to the relevant spec then? Admittedly my intuition is a year stale at the moment, but I went looking hard for this at one point and found nothing other than source code.
No spec, because technical details are complex, but spec, when it is written, will just be an elaboration of "It is an error to access a place, when an access conflicts with a loan, and the loan is live". This has never changed since Rust 1.0, more than 5 years. Spec will just need to define "place" and "live" and that takes time.