Engines history: Why is rails-3.2 engines guide accessible but not listed in 3.2 guides? How did you learned out about engines in Rails 3?

Good morning everyone!
I research Ruby on Rails engines for my master thesis. But I am confused a bit after exploring the docs and Rails repository. First I can easily find the “rails 3 engines guide” for the 3.2.13 version via google, but this guide is not accessible via guides list for 3.2.13. Did docs maintainers forget to destroy this page?
Also how did you learned out about engines in Rails 3? What was the main source of information (guides, news etc.) for Rails 3 engines? (I am not sure everybody explored Ruby on Rails repository)
Can you say that Rails engines were “fully” released in Rails 3.0 or Rails 3.1?