Correct documentation of VAVReheat models

[mwetter]

The VAVReheat model uses the base class Buildings.Examples.VAVReheat.BaseClasses.PartialOpenLoop. This base class implements a replaceable model that contains the structure of the envelope model. PartialOpenLoop states that this model is representative for the medium office building. However, this description should be done in the actual implementation of the medium office model, rather than in PartialOpenLoop, in particular because PartialOpenLoop is also used in the Spawn examples which replace the model with a small office building.