documenting ruby doc

I am looking for ways to document ruby classes, functions, etc.
All the references take me to RDOC but the project on source forge has
been marked inactive.
Does anybody know what to use, how to use or I just need to write
comments above class/function.


You use RDoc. It's part of Ruby now. Issues are being tracked at Ruby
Forge not Source Forge anymore for RDoc.

It's already stable and widely used enough that you can find
references anywhere on how to use it. Here's a link from wikibooks to
help you get started.