Added info about automatic discriminator map

see https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L257
Merge pull request #975 from tkleinhakisa/doc-automatic-discriminator-map

Added info about automatic discriminator map
Adding note about DDC-3020 - external XML entities may or may not work
Merge branch 'docs/DDC-3020'
Fix wrong annotation

Without above fix I'm getting 
"[Semantical Error] The annotation "@array" in method Doctrine\ORM\Mapping\ClassMetadataInfo::mapEmbedded() was never imported. Did you maybe forget to add a "use" statement for this annotation?"
Merge pull request #977 from mvrhov/patch-2

Fix wrong annotation