Gem::RDoc#setup

# File tmp/rubies/ruby-3.3.0/lib/rdoc/rubygems_hook.rb, line 239
def setup
  self.class.load_rdoc

  raise Gem::FilePermissionError, @doc_dir if
    File.exist?(@doc_dir) and not File.writable?(@doc_dir)

  FileUtils.mkdir_p @doc_dir unless File.exist? @doc_dir
end