Code is the end result of a thinking / problem solving process, not the process itself. If I implement a mathematical formula in code, the “documentation” is the academic paper where the formula is described and proved, and the whole context of textbooks and other papers where the relevant terms are defined and abstractions are constructed, not the handful of lines of abstract arithmetic on one-letter variable names in the code. If I throw away the paper, the code doesn’t suddenly become self-documenting.

If you’re going to make such a reductionist definition of “documentation”, why not go all the way: every executable program is just a sequence of clear and simple instructions to the machine. The documentation of what each instruction does is right there in black and white. Why would you need anything else? All programs are documented.