Clarify query_builder closure return type

It wasn't clear what the Closure should return when passing it in the `query_builder` option.

Author: kix

reference/forms/types/entity.rst

@@ -165,7 +165,7 @@ If specified, this is used to query the subset of options (and their
 order) that should be used for the field. The value of this option can
 either be a ``QueryBuilder`` object or a Closure. If using a Closure,
 it should take a single argument, which is the ``EntityRepository`` of
-the entity.
+the entity and return an instance of ``QueryBuilder``.
 
 Overridden Options
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