updating page for 'uri documentation discovery' product

I've prepared

 http://www.w3.org/2001/tag/products/defininguris-2012-02-11.html

as an update to

 http://www.w3.org/2001/tag/products/defininguris.html

pushing dates out one month, adding some more 'success criteria', and
organizing the document lists differently.

Not sure what our process is around updating product pages. I hope
it's lightweight but I'll leave this where it is until I find out.

FWIW I updated the product name and dates in products/index.html.

Jonathan