Documentation Change: add controller enabled to test helper

Not sure if this is an appropriate change, but it took me quite a while to figure out why one particular model test was failing in the suite but not individually.

If your controller tests load before your model tests the test.rb
config will cause model test versions to not work even after setting
PaperTrail.enabled to true.

See:
https://github.com/bjallen/paper_trail/blob/6de04f199e7c6c885f922c231872d04564fb931c/lib/paper_trail/has_paper_trail.rb#L391