[cema]

A self-documenting code means we can (relatively) easily see what is being done by the program just by reading the program.

What is one question; why is another. When we work with the existing code (debugging, maintenance) we need to know answers to both questions. So, in addition to a well written (self-documenting) code base we need a document explaining the rationale for the choices that were made and, more generally, the context in which the code was created.