message: Update activations.py
  improve documentation of sigmoid function
diff: @@ -274,6 +274,8 @@ def sigmoid(x):
  >>> a = tf.constant([-20, -1.0, 0.0, 1.0, 20], dtype = tf.float32)
  >>> b = tf.keras.activations.sigmoid(a)
  + >>> b.numpy()
  + [2.0611537e-09, 2.6894143e-01, 5.0000000e-01, 7.3105860e-01, 1.0000000e+00]
  >>> b.numpy() >= 0.0
  array([ True, True, True, True, True])
  