Add API documentation for public class attributes

It would be better for API documentation if "public" class attributes, e.g. models.Doc2Vec.docvecs, were explicitly documented as part of the class with examples describing appropriate use. Hopefully, this would help mitigate questions such as this one.

This could be achieved in a few ways with Sphinx:

1. Via __init__ doc strings with autoattribute (example)
2. Use Python properties