The API documentation is probably not where you want to look in this instance. It's more intended for looking up the behaviour or options for a specific method call.

http://guides.rubyonrails.org is the equivalent of the Python documentation you point to for Rails.

If you want Ruby-specific documentation, there are versions of the Pickaxe book online (http://phrogz.net/programmingruby/). Admittedly old versions, but well organised by-topic nonetheless.