Test double documentation tweaks

Created by: jamesmartin

Added a high level description of test doubles and the similarity between mock, stub and double methods. The old "when to use a mock, spy, stub, double, fake, etc. etc." question comes up quite a lot and I wanted something to point to when discussing these in the context of RSpec.

Also a small cuke feature addition to document defining multiple methods/return values as arguments to double.

My first pull request, so hopefully if this works more substantial updates will follow :)