Gem::Commands::SetupCommand#install_rdoc

# File tmp/rubies/ruby-3.1.3/lib/rubygems/commands/setup_command.rb, line 321
def install_rdoc
  gem_doc_dir = File.join Gem.dir, "doc"
  rubygems_name = "rubygems-#{Gem::VERSION}"
  rubygems_doc_dir = File.join gem_doc_dir, rubygems_name

  begin
    Gem.ensure_gem_subdirectories Gem.dir
  rescue SystemCallError
    # ignore
  end

  if File.writable?(gem_doc_dir) &&
     (!File.exist?(rubygems_doc_dir) ||
      File.writable?(rubygems_doc_dir))
    say "Removing old RubyGems RDoc and ri" if @verbose
    Dir[File.join(Gem.dir, "doc", "rubygems-[0-9]*")].each do |dir|
      rm_rf dir
    end

    require_relative "../rdoc"

    fake_spec = Gem::Specification.new "rubygems", Gem::VERSION
    def fake_spec.full_gem_path
      File.expand_path "../../..", __dir__
    end

    generate_ri   = options[:document].include? "ri"
    generate_rdoc = options[:document].include? "rdoc"

    rdoc = Gem::RDoc.new fake_spec, generate_rdoc, generate_ri
    rdoc.generate

    return true
  elsif @verbose
    say "Skipping RDoc generation, #{gem_doc_dir} not writable"
    say "Set the GEM_HOME environment variable if you want RDoc generated"
  end

  return false
end