message: Add a testable example to tf.math.square
diff: @@ -6,4 +6,11 @@ op {
  endpoint {
  name: "square"
  }
  + description: <<END
  +I.e., \\(y = x * x = x^2\\).
  +
  +>>> tf.math.square([-2., 0., 3.])
  +<tf.Tensor: shape=(3,), dtype=float32, numpy=array([4., 0., 9.], dtype=float32)>
  +
  +END
  }
  