[R-package] package documentation using roxygen2 7.0.0+

[jameslamb]

roxygen2 7.0.0 was recently released to CRAN. To close this issue, install roxygen2 locally and then change the R package's code so that the following works:

export CXX=/usr/local/bin/g++-8
export CC=/usr/local/bin/gcc-8
Rscript build_r.R
cd R-package
Rscript -e "roxygen2::roxygenize(load = 'installed')"

[jameslamb]

closing in favor of being in #2302 . If you'd like to pick this up, comment on the issue and we will open it back up!

[jameslamb]

opening this as I think we need to fix it for #2530