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 114
def generate_ri
setup_rdoc
install_ri # RDoc bug, ri goes first
FileUtils.mkdir_p @doc_dir unless File.exist?(@doc_dir)
end