Ruby: run 'ri socket::accept' get 'no matches found'

[dreamrider2011]

I have use rvm install ruby 2.1.1, and gem install rails 4.0.1.
But I can just use ri to get rails' help not ruby.
such as ri links_to will success, but socket::accept not.
I have gem install rdoc and rdoc-data 4.1.1
thanks.