Yeah, ruby-doc.org is pretty annoying in this regard, especially since they do actually have the stdlib documentation available, they just don't link to it. Have a look at this search, for example:
I think that is only core, though, not the standard lib. It started for me with hearing the rumor that JSON would be part of the standard lib of Ruby 1.9. I was unable to confirm that online. Eventually I peeked into the "Programming Ruby" book in a bookshop and thus was able to confirm that Ruby 1.9.1 indeed comes with JSON.
http://www.google.com/search?q=site%3Aruby-doc.org%2Fcore-1....
There's the documentation for the Array class, but you can't get to it from the 1.9 core documentation:
http://ruby-doc.org/core-1.9/index.html