[DoctrineBridge] Document the new entityClass option

Author: ogizanagi

reference/constraints/UniqueEntity.rst

@@ -12,6 +12,7 @@ using an email address that already exists in the system.
 |                | - `message`_                                                                        |
 |                | - `em`_                                                                             |
 |                | - `repositoryMethod`_                                                               |
+|                | - `entityClass`_                                                                    |
 |                | - `errorPath`_                                                                      |
 |                | - `ignoreNull`_                                                                     |
 |                | - `payload`_                                                                        |
@@ -164,6 +165,19 @@ The name of the repository method to use for making the query to determine
 the uniqueness. If it's left blank, the ``findBy`` method will be used.
 This method should return a countable result.
 
+entityClass
+~~~~~~~~~~~
+
+**type**: ``string`` **default**: ``null``
+
+.. versionadded:: 3.2
+    The ``entityClass`` option was introduced in Symfony 3.2.
+
+The fully-qualified class name (FQCN) of the entity to get the repository used for making the query to determine
+the uniqueness. If it's left blank, the exact entity instance repository will be used.
+If set, the selected entity repository will be used regardless of the real class of the instance.
+It's particularly useful when dealing with Doctrine Inheritance Mapping.
+
 errorPath
 ~~~~~~~~~