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* [R-package] deprecate lgb.prepare() and lgb.prepare2()

* linting

* renaming

* updated docs

* linting

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* one more comment fix

* remove comment about int being dangerous

* empty commit
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>