There's nothing wrong with what your parent said. If we are speaking of API documentation (i.e. function or module documentation): The "why" can't be explained

There's nothing wrong with what your parent said. If we are speaking of API documentation (i.e. function or module documentation): The "why" can't be explained here. It's at the caller side. There are more rare cases where it's reasonable to document the implementation of a function because there's a particularly tricky section of code -- and documenting the "why" makes more sense to me here.