Document getPartialReference() properly
Document getPartialReference() properly

According to the current implementation that method also returns `null`,
however the interface's documentation was incorrect.

Majkl578/doctrine2,Majkl578/doctrine2,aivus/doctrine2,lcobucci/doctrine2,TomasVotruba/doctrine2,aivus/doctrine2,lcobucci/doctrine2,TomasVotruba/doctrine2,holtkamp/doctrine2,holtkamp/doctrine2