You can add
\keyword{internal}
to the Rd file. Your documentation won't show up the in the pdf manual, it
won't show up in the package index, but you'll still be able to access the
doc page with ?levels.no or help("levels.no").
This is usually used in a package's deprecated and defunct doc pages
Dear package developeRs,
I am working on fixing some notes regarding package DoE.base.
One note refers to the function levels.no and complains that the
function is not documented as a method for the generic function levels.
Actually, it is not a method for the generic levels, but a standalone
i
