Question: And where can I find it? I'm running OSX.

In addition I have to do a lot of work offline while travelling so if anyone can offer any tips for downloadable documentation please let me know.

Answer: 
If you have installed your gems the "normal" way, they install rdoc documentation.

Just run `gem server` in your console and go to <http://localhost:8808> in your browser. There you'll find all available Gems and the RDoc documentation
@klaustopher's answer didn't work for me directly, so here is how I got this to work for the rspec-expectations gem:

1. Install gem documentation by either

	* using the `--rdoc` flag: `gem install --rdoc rspec-expectations`
	* or by installing rdocs for all gems: `gem rdoc --all`
2. Run rdoc server.

`gem server`

3. Go to `http://localhost:8808` in my browser.

Caveat: at a first glance the rdoc for `rspec-expectations` is pretty useless. I bet there is a better way to do this.