Clarify in rdoc comments what the public api of pickle actually does

This got lost because I hadn't realised lighthouse keeps ardes and ianwhite separate ticket wise.

Some quick notes

How do I look up one pickled model?

model(the model reference)

e.g. model("the page")

How do I look up all the pickled models?

created_models(factory)

Things to document:

• what the expected inputs are for 'factory' 'model' 'name' etc in the api, i.e. what I can feed it to get something back
• how find_models and created_models are different (one finds a model in the db, the other uses the index)