message: Added example related to top_k parameter in tf.keras.metrics.Precision
diff: @@ -1191,6 +1191,17 @@ class Precision(Metric):
  >>> m.result().numpy()
  1.0
  + >>> # With top_k=2, it will calculate precision over y_true[:2] and y_pred[:2]
  + >>> m = tf.keras.metrics.Precision(top_k=2)
  + >>> _ = m.update_state([0, 0, 1, 1], [1, 1, 1, 1])
  + >>> m.result().numpy()
  + 0.0
  + >>> # With top_k=2, it will calculate precision over y_true[:4] and y_pred[:4]
  + >>> m = tf.keras.metrics.Precision(top_k=4)
  + >>> _ = m.update_state([0, 0, 1, 1], [1, 1, 1, 1])
  + >>> m.result().numpy()
  + 0.5
  +
  Usage with tf.keras API:
  ```python
  