It's not easy to make fixed rules about these things, but I do believe that too little documentation is the enemy of maintainable code. In the end we have to rely on the professionalism and common sense of the programmer. If (s)he doesn't have any we're doomed anyway.
I think I'm generally in agreement with what you say. I advocate the implementation of 'user stories' as high-level code, if only as a set of single-use functions, to match (where possible) the project brief supplied by the client. Then it's quite clear what the intention of the program is without the need for excessive documentation. That tends to answer the "why?" question that the code itself rarely reveals.
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