It is not referring to mandb(8), but to the more discursive manual for the package as a whole that you will find in /usr/share/doc/man-db/man-db-manual.{ps,txt}.gz. I'll have a think about how to make this more clearly structured; it might be that the right answer is to migrate the package manual into a section 7 manual page, as it's a bit awkwardly laid out right now.
It is not referring to mandb(8), but to the more discursive manual for the package as a whole that you will find in /usr/share/ doc/man- db/man- db-manual. {ps,txt} .gz. I'll have a think about how to make this more clearly structured; it might be that the right answer is to migrate the package manual into a section 7 manual page, as it's a bit awkwardly laid out right now.