Document that AUTOGENERATE_ constants are allowed

As of Doctrine Common 2.4, there are four strategies for generating proxy classes. Previously there were just two. This is supported by Doctrine ORM (implemented in bee74f898da0474b4bad44d41df84f1807036880 and 88754622419524bf92cfc18f9ab7fac148c35924), but the change is not fully reflected in the documentation and variable names used in Doctrine ORM.

This PR updates the documentation and variable names.