Although this question is old, nobody has yet provided a proper solution. I just ran into the same issue, and found the solution:
If you are using RVM:
rvm docs generate-ri # <- Just the ri docs, much faster
rvm docs generate # <- Everything (rdoc + ri)
More info on managing RVM docs here:
https://rvm.io/rubies/docs
If not using RVM:
gem install rdoc-data
# Regenerate system docs
rdoc-data --install
# Regenerate all gem docs (rdoc + ri)
gem rdoc --all --overwrite
# Regenerate all gem docs (ri only)
gem rdoc --all --overwrite --ri --no-rdoc
# Regenerate specific gem doc with specific version
gem rdoc gemname -v 1.2.3 --overwrite