docs: add JSONB containment example of a key and empty object

Backpatch through 9.5

doc/src/sgml/json.sgml

@@ -291,6 +291,9 @@ SELECT '[1, 2, [1, 3]]'::jsonb @> '[[1, 3]]'::jsonb;
 
 -- Similarly, containment is not reported here:
 SELECT '{"foo": {"bar": "baz"}}'::jsonb @> '{"bar": "baz"}'::jsonb;  -- yields false
+
+-- A top-level key and an empty object is contained:
+SELECT '{"foo": {"bar": "baz"}}'::jsonb @> '{"foo": {}}'::jsonb;
 </programlisting>
 
   <para>