"Documentation by paper"

[Richard Kenner]

    That's not a formal specification, but an informal description, just
    as we have in our comments right now.  In fact, our current comment
    goes into a little detail than that, although it is still necessarily
    incomplete.

We're talking about "informal description".  The term "specification" is not
a shorthand for "formal specification", but for "interface specification".

    You couldn't use either of these descriptions to tell you all you need
    to implement the function separately.

Of course not, but that's not its purpose.  The documentation of the
interface says what you have to know to *call* the function, not to
implement it.