Document getPartialReference() properly
Document getPartialReference() properly

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

Ref: https://github.com/doctrine/doctrine2/blob/v2.6.2/lib/Doctrine/ORM/EntityManager.php#L514-L516

bigfoot90/doctrine2,doctrine/doctrine2,Sitetheory/doctrine2,bigfoot90/doctrine2,doctrine/doctrine2,Ocramius/doctrine2,Ocramius/doctrine2,Sitetheory/doctrine2,Ocramius/doctrine2