[#6594] Add versionadded directive and capitialize Closure

Author: wouterj

reference/forms/types/entity.rst

@@ -202,10 +202,14 @@ query_builder
 Allows you to create a custom query for your choices. See
 :ref:`ref-form-entity-query-builder` for an example.
 
-The value of this option can either be a ``QueryBuilder`` object, a Closure or ``null``.
-When using a Closure, you will be passed the ``EntityRepository`` of the entity
-as the only argument and should return a ``QueryBuilder``.  
-If you'd like to display a list of empty entries, you can return ``null`` in the closure.
+The value of this option can either be a ``QueryBuilder`` object, a Closure or
+``null``.  When using a Closure, you will be passed the ``EntityRepository`` of
+the entity as the only argument and should return a ``QueryBuilder``. If you'd
+like to display an empty list of entries, you can return ``null`` in the
+Closure.
+
+.. versionadded:: 3.1
+    Returning ``null`` in the Closure was introduced in Symfony 3.1.
 
 Overridden Options
 ------------------