I don't see how asking AI to write some description of why it wrote this or that code would actually result in an explanation of why it wrote that code? It's not like it's thinking about it in that way, it's just generating both things. I guess they'd be in the same context so it might be somewhat correct.
Have you tried it? LLMs are quite good at summarizing. Not perfect, but then neither are humans.
If you ask it to document why it did something, then when it goes back later to update the code it has the why in its context. Otherwise, the AI just sees some code later and has no idea why it was written or what it does without reverse engineering it at the moment.