Yes, the suggestion was just for the docs, and since those are intended for human rather than machine consumption, it's fine if there are some blurry cases where there is no signature.  Ideally in those cases, the issue should be explained in the doc text.

I actually don't see what's wrong with including signatures in the source code as well, as long as doing so doesn't break anyone's existing code.  I agree with Veky that one should be very hesitant about breaking existing working code, even if that code relies on undocumented behavior.