I've never been massively keen on the carious implementations of ruby, they're all a bit different and those differences appear to be documented poorly. This is most visible if compared to the python landscape.
0: https://github.com/rubyspec/rubyspec 1: http://rvm.beginrescueend.com/
My understanding is that the rubyspec project (http://rubyspec.org/), is an effort to retrospectively document all the stuff the language does.
Compare ruby: http://eigenclass.org/hiki.rb?Changes+in+Ruby+1.9, to python: http://docs.python.org/dev/whatsnew/2.7.html
I appreciate this isn't a great example, as python 2.7 isn't a different implementation (ruby 1.8 -> 1.9 is though), but this has been my experience of the way things are documented language-wise.