Note that if both RI and RDoc documents are generated from the same process,
the RI docs should be done first (a likely bug in RDoc will cause RI docs
generation to fail if run after RDoc).
# File lib/rubygems/doc_manager.rb, line 128
def generate_rdoc
setup_rdoc
install_rdoc
FileUtils.mkdir_p @doc_dir unless File.exist?(@doc_dir)
end