Added hint about exception in Query#getOneOrNullResult()
Added hint about exception in Query#getOneOrNullResult()

When calling `Query#getOneOrNullResult()` and there are more than one
objects in the result an `NonUniqueResultException` is thrown.
This information was missing in the documentation about the query result
formats.

This commit addresses #6793.

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