[Rd] check for \value in R CMD check

[Ivan Krylov]

This was discussed a bit in July 2025:
https://stat.ethz.ch/pipermail/r-package-devel/2025q3/011824.html

Part of the problem with an automatic check is that it can be easily
circumvented by adding a one-line value section (as many packages are
wont to do, "#' @return data.frame" with no further context), so human
review will still be needed.

It's probably worth adding such a check, initially with a level below
-1, so that it's not reported by R CMD check by default and doesn't
blow up CRAN.

-- 
Best regards,
Ivan