minor #6599 Fixed null description of query_builder option (HeahDude)

This PR was merged into the 2.8 branch.

Discussion
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Fixed null description of query_builder option

doc fix: 2.8+

ref symfony/symfony#13990 (comment)

Commits
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14482e4 Fixed null description of query_builder option

Author: wouterj

reference/forms/types/entity.rst

@@ -207,8 +207,8 @@ The value of this option can either be a ``QueryBuilder`` object, a Closure or
 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.
+If you'd like to use default loading of all entries, you can return ``null`` in
+the Closure as well.
 
 .. versionadded:: 2.8
     Returning ``null`` in the Closure was introduced in Symfony 2.8.