For Firefox, you must use the plugin "XHTML Ruby Support" (https://addons.mozilla.org/en-US/firefox/addon/1935
For IE, it work natively.
As I know, don't work with WebKit based browser (Google chrome and Safari), but I don't know if there is any plugin.
For examples (and test in you browser) look at this page: http://www.w3.org/International/tests/sec-ruby-markup-1
The example with the parenthesis (as the <rp> value) is in the point "3 Simple ruby, short, wide-cell ruby text".
Of course, you need to be able to see Japanese characters in your computer to see it properly.
Try this code in your wiki (or dokuwiki playground)
If your browser render the ruby, you will see "Normal text" with a small "Ruby text" over.
If it did not work you will see "Normal text (Ruby text)"