There is however the problem that historically they have not keep a copy of the old docs online. So if you stick to 2.2.2 your probably going to be SOL when it comes to reference info.
Sure there is an archive somewhere of the old docs but that is a long way from ideal.
Aren't the docs part of the repo?
Yes they are. Get them here:
https://github.com/twitter/bootstrap/tree/master/docs
There's nothing wrong with intentionally staying behind -- Bootstrap 3.x features will more than likely lean a lot more heavily on CSS3/HTML5.