See: https://rvm.io/rubies/docs/
In order to conserve space, RVM does not automatically generate and install each Ruby's ri / rdoc documentation.
Provided you have not cleaned up the extracted sources for all currently installed Rubies by executing 'rvm cleanup all' then you can install the docs for all currently installed Rubies by executing:
rvm all do rvm docs generate all
Now execute:
ri Array#map
Or if on zsh:
ri 'Array#map'
Success!