On “Chapter 1. Getting started”, on footnote #10 it suggests to run
rvm docs generate to generate ri compatible documentation if you have rvm.
In my case (rvm installed for a single user on Mountain Lion, ruby-2.0.0-p0) I had to install the rdoc-data gem:
$ gem install rdoc rdoc-data $ rdoc-data --install
Maybe the last step isn’t necessary.
Without rdoc and rdoc-data I would get something like this:
$ ri GC.disable Nothing known about GC