Minor rephrasing of some comment in a code example, to be clearer

doc/extending/unittest.txt

@@ -398,7 +398,9 @@ Here is an example showing how to use verify_grad:
 >>>    # cause verify_grad to crash.
 >>>    op = tensor.nnet.crossentropy_softmax_argmax_1hot_with_bias
 >>>    def op_with_fixed_y_idx(x, b):
->>>        # Note that this op has multiple outputs: we need to pick only one.
+>>>        # Although this op has multiple outputs, we can return only one.
+>>>        # Here, we return the first output only, and fix the value of the
+>>>        # `y_idx` input to some constant array.
 >>>        return op(x, b, y_idx=numpy.asarray([0, 2]))[0]
 >>>    x_val = numpy.asarray([[-1, 0, 1], [3, 2, 1]], dtype='float64')
 >>>    b_val = numpy.asarray([1, 2, 3], dtype='float64')