[R-pkg-devel] check --as-cran error on Rd file

blurfle$ cat sped.Rcheck/00check.log
* using log directory ?/home/geyer/Software/sped/package/sped.Rcheck?
* using R Under development (unstable) (2020-03-10 r77920)
* using platform: x86_64-pc-linux-gnu (64-bit)
* using session charset: UTF-8
* using option ?--as-cran?
* checking for file ?sped/DESCRIPTION? ... OK
* this is package ?sped? version ?0.1?
* checking CRAN incoming feasibility ... NOTE
Maintainer: ?Charles J. Geyer <charlie at stat.umn.edu>?

New submission
* checking package namespace information ... OK
* checking package dependencies ... OK
* checking if this is a source package ... OK
* checking if there is a namespace ... OK
* checking for executable files ... OK
* checking for hidden files and directories ... OK
* checking for portable file names ... OK
* checking for sufficient/correct file permissions ... OK
* checking serialization versions ... OK
* checking whether package ?sped? can be installed ... OK
* checking installed package size ... OK
* checking package directory ... OK
* checking for future file timestamps ... OK
* checking DESCRIPTION meta-information ... OK
* checking top-level files ... OK
* checking for left-over files ... OK
* checking index information ... OK
* checking package subdirectories ... OK
* checking R files for non-ASCII characters ... OK
* checking R files for syntax errors ... OK
* checking whether the package can be loaded ... OK
* checking whether the package can be loaded with stated dependencies ... OK
* checking whether the package can be unloaded cleanly ... OK
* checking whether the namespace can be loaded with stated dependencies ...
OK
* checking whether the namespace can be unloaded cleanly ... OK
* checking loading without being on the library search path ... OK
* checking use of S3 registration ... OK
* checking dependencies in R code ... OK
* checking S3 generic/method consistency ... OK
* checking replacement functions ... OK
* checking foreign function calls ... OK
* checking R code for possible problems ... OK
* checking Rd files ... OK
* checking Rd metadata ... OK
* checking Rd line widths ... OK
* checking Rd cross-references ... OK
* checking for missing documentation entries ... OK
* checking for code/documentation mismatches ... WARNING
Variables with usage in documentation object 'alberta' but not in code:
   ?alberta?

* checking Rd \usage sections ... OK
* checking Rd contents ... OK
* checking for unstated dependencies in examples ... OK
* checking contents of ?data? directory ... OK
* checking data for non-ASCII characters ... OK
* checking data for ASCII and uncompressed saves ... OK
* checking line endings in C/C++/Fortran sources/headers ... OK
* checking line endings in Makefiles ... OK
* checking compilation flags in Makevars ... OK
* checking for GNU extensions in Makefiles ... OK
* checking for portable use of $(BLAS_LIBS) and $(LAPACK_LIBS) ... OK
* checking use of PKG_*FLAGS in Makefiles ... OK
* checking use of SHLIB_OPENMP_*FLAGS in Makefiles ... OK
* checking pragmas in C/C++ headers and code ... OK
* checking compilation flags used ... OK
* checking compiled code ... OK
* checking examples ... OK
* checking for unstated dependencies in ?tests? ... OK
* checking tests ... OK
   Running ?foo.R?
* checking PDF version of manual ... OK
* checking for non-standard things in the check directory ... OK
* checking for detritus in the temp directory ... OK
* DONE
Status: 1 WARNING, 1 NOTE
blurfle$

I do not get this warning when --as-cran is omitted.  I have never seen the
warning before.  The Rd file is

blurfle$ cat sped/man/alberta.Rd
\name{alberta}
\docType{data}
\alias{alberta}
\title{Pedigree Data on Equus przewalskii in Alberta}
\description{
   Pedigree of Asian wild horse (also called Mongolian wild horse)
   \emph{Equus przewalskii} individuals living in Alberta, Canada in 1988.
}
\usage{alberta}
\format{
A matrix with 69 rows giving (individual, father, mother) triplets.
The column labels are \code{ind}, \code{pa}, and \code{ma}.
The names (which are numbers) are studbook numbers.
There are 82 individuals in this pedigree in all.
There are \eqn{82 - 69 = 13} founders.
}
\examples{
library(sped)
data(alberta)
}
\keyword{datasets}
blurfle$

And this is AFAICS what Section 2.1.2 of Writing R Extensions (R-devel
version) says it should be.

So what is this warning about?