Nothing known about.... when trying ri String#upcase Ruby
RDoc is installed, I'm not sure why that particular lookup does not work. Perhaps somebody else can shed some light on that.
However, if you just need to find out about how to use particular parts of the API, you can't go wrong with a google search. All of the Rdoc documentation is available online from numerous locations.
Here's String#upcase for example: http://ruby-doc.org/core/classes/String.html#M001155