Added info about automatic discriminator map

see https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php#L257

docs/en/reference/inheritance-mapping.rst

@@ -123,6 +123,9 @@ Things to note:
    be fully qualified if the classes are contained in the same
    namespace as the entity class on which the discriminator map is
    applied.
+-  If no discriminator map is provided, then the map is generated
+   automatically. The automatically generated discriminator map 
+   contains the lowercase short name of each class as key.
 
 Design-time considerations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -210,6 +213,9 @@ Things to note:
    be fully qualified if the classes are contained in the same
    namespace as the entity class on which the discriminator map is
    applied.
+-  If no discriminator map is provided, then the map is generated
+   automatically. The automatically generated discriminator map 
+   contains the lowercase short name of each class as key.
 
 .. note::