How can I install RDOC of gems already installed?

I have some gems that don't have ri/rdoc files and I'd like to know if ther开发者_运维知识库e is a way to install this rdoc files.

---

EDIT:

Some of the answers on this question may help you.

OLD:

This site seems to have your answer.

Example:

gem rdoc rake --version 0.5.4

---

Just rerun (works for gem v1.8.23):

gem install your_gem_name --rdoc