message: Add usage example for tf.math.sigmoid
  Add a brief example showing how to use the sigmoid function in tf.math
diff: @@ -3213,6 +3213,12 @@ def sigmoid(x, name=None):
  Returns:
  A Tensor with the same type as `x`.
  + Usage Example:
  +
  + >>> x = tf.constant([-128.0, 0.0, 128.0], dtype=tf.float32)
  + >>> tf.sigmoid(x)
  + <tf.Tensor: shape=(3,), dtype=float32, numpy=array([0. , 0.5, 1. ], dtype=float32)>
  +
  @compatibility(scipy)
  Equivalent to scipy.special.expit
  @end_compatibility
  