[R-package] [docs] upgrade docs to roxygen2==7.2.0

[jameslamb]

Contributes to #3763.

{roxygen2} v 7.2.0 was released 16 days ago (https://cran.r-project.org/web/packages/roxygen2/index.html). This project currently uses v7.1.2.

This PR proposes updating the R package's documentation to use the newest version of {roxygen2}.

I regenerated the R package's documentation by running the following.

# install latest roxygen2 locally
Rscript -e "install.packages('roxygen2', repos = 'https://cran.r-project.org')"

# rebuild docs
sh build-cran-package.sh --no-build-vignettes
R CMD INSTALL --with-keep.source ./lightgbm_3.3.2.tar.gz
cd R-package/
Rscript -e "roxygen2::roxygenize(load = 'installed')"

Notes for Reviewers

I didn't see this item listed in #3763, so I edited the description there today to include it.

@StrikerRUS could you temporarily enable readthedocs builds for this branch so we can check that this change will work there?

[StrikerRUS]

LGTM!

Docs look correct: https://lightgbm.readthedocs.io/en/docs-roxygen2-version/R/index.html

[jameslamb]

excellent, thanks @StrikerRUS !

[github-actions]

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