Patch 1

Adding clarification that the benefits of the "Matt trick" for hierarchical models only works when there is a lot of data making it worth shifting the data's correlation from the parameters to the hyperparameters. See https://groups.google.com/d/msg/stan-users/dFFTF0n6mYo/WN3vWOf9ktAJ and https://groups.google.com/d/msg/stan-users/dFFTF0n6mYo/lTIfqVN8AB8J.