Using Spring was hiding an loading error. When you start Rspec, Rails
and its engines are not loaded yet. So our way to load the spec helper
via `Rails.root` did not work when you ran specs on their own without
loading Rails with Spring first.
This allows us to run the specs separately to generate the
documentation. It's more efficient this way and the separate swagger doc
file is easier to read.
The engine-specific swagger helper also allows us to simplify the spec
files.
Added an exception to our styleguide because it's intended and useful to
have a complete (lengthy) description of the API in one block.
