minor #6596 Fixed query_builder option (HeahDude)

This PR was merged into the 2.3 branch.

Discussion
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Fixed query_builder option

doc fix: 2.3+

ref #6594 (comment)

Commits
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45f586b Fixed query_builder option

reference/forms/types/entity.rst

@@ -166,12 +166,13 @@ cast into a string and so must have a ``__toString()`` method.
 query_builder
 ~~~~~~~~~~~~~
 
-**type**: ``Doctrine\ORM\QueryBuilder`` or a Closure
+**type**: ``Doctrine\ORM\QueryBuilder`` or a Closure **default**: ``null``
 
 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 or a Closure.
+The value of this option can either be a ``QueryBuilder`` object, a Closure or
+``null`` by default which loads all entities.
 When using a Closure, you will be passed the ``EntityRepository`` of the entity
 as the only argument and should return a ``QueryBuilder``.