Uh, it's simple .... No, it's not easy. But it's absolutely knowable and testable.
I agree that it's completely doable, but the poster I replied to was stating that the programmer who wrote the algorithm must understand how it's making decisions and that only the less skilled maintenance coders would be confused. That's simply not true. I know people who could write a neural net from a reasonable spec but doing the steps you described above would blow their minds. I'd also argue that a NN with even a few layers of nodes can get complex fast enough that what you're proposing would result in a document the size of a novel and still not capture all the nuances.
I really appreciate your point that
Getting any useful info out of that will be an issue though. You may find out that somewhere deep in your neural net it's looking for a seemingly random pattern of contrast or checking against some strange distance/angle.
If the net is using some seemingly random pattern that's where you can get some bizarre (to human thinking) failures. We tend to understand when something goes wrong in a way we can comprehend. If the seemingly random pattern the computer finds happens to call a slightly obscured "stop sign" a "no u-turn" sign that would be incomprehensible to a human, but might make perfect sense to the NN.
This all isn't to say that you can't reduce the odds of this sort of problem to such a small number that it's meaningless especially in comparison to human error. Still, when crap like this happens it makes the news and gets blown all out of proportion, so expect "the sky is falling" stories to follow any uncertainty AI behavior.