Added hint about exception in Query#getOneOrNullResult()

When calling `Query#getOneOrNullResult()` and there are more than one
objects in the result an `NonUniqueResultException` is thrown.
This information was missing in the documentation about the query result
formats.

This commit addresses doctrine#6793.

docs/en/reference/dql-doctrine-query-language.rst

@@ -996,8 +996,9 @@ the Query class. Here they are:
    result contains more than one object, an ``NonUniqueResultException``
    is thrown. If the result contains no objects, an ``NoResultException``
    is thrown. The pure/mixed distinction does not apply.
--  ``Query#getOneOrNullResult()``: Retrieve a single object. If no
-   object is found null will be returned.
+-  ``Query#getOneOrNullResult()``: Retrieve a single object. If the
+   result contains more than one object, an ``NonUniqueResultException``
+   is thrown. If no object is found null will be returned.
 -  ``Query#getArrayResult()``: Retrieves an array graph (a nested
    array) that is largely interchangeable with the object graph
    generated by ``Query#getResult()`` for read-only purposes.