ri binary package in Ubuntu Xenial amd64

 ri is a command line tool that displays descriptions of built-in Ruby
 methods, classes, and modules. For methods, it shows you the calling
 sequence and a description. For classes and modules, it shows a
 synopsis along with a list of the methods the class or module
 implements.
 .
 This package provides ri command and descriptions about Ruby.
 This package provides the ri documentation for the Ruby standard library.
 The ri executable itself is in the ruby package.
 .
 This package is a dependency package, which depends on Debian's default Ruby
 version (currently v2.3).

Source package