There are several forms of documentation:

  * comments in code
  * team-based comments
  * project design docs
  * knowledge-base articles for handling on-call rotation around feature
  * how-to guides for customers

Each of these has a different cost and a different direct/external usefulness. I absolutely believe in good documentation and I absolutely believe that it's valuable. It does not negate the extra cost of including these forms of documentation - especially the "not-in-code" documentation.