Comment in description of test for String.replace()

Author: Joel Allred

regression/strings-smoke-tests/java_replace/test.desc

@@ -6,3 +6,5 @@ test_replace.class
 ^VERIFICATION SUCCESSFUL$
 --
 diffblue/test-gen#256
+This test is only effective with a model of
+java.lang.String.replace(LCharSequence; LCharSequence).