message: Update docs to add note and examples for tf.count_nonzero with string
diff: @@ -1466,9 +1466,18 @@ def count_nonzero(input_tensor,
  tf.count_nonzero(x, [0, 1]) # 3
  ```
  + **NOTE** Strings are compared against zero-length empty string `""`. Any
  + string with a size greater than zero is already considered as nonzero.
  +
  + For example:
  + ```python
  + x = tf.constant(["", "a", " ", "b", ""])
  + tf.count_nonzero(x) # 3, with "a", " ", and "b" as nonzero strings.
  + ```
  +
  Args:
  - input_tensor: The tensor to reduce. Should be of numeric type, `string`,
  - or `bool`.
  + input_tensor: The tensor to reduce. Should be of numeric type, `bool`,
  + or `string`.
  axis: The dimensions to reduce. If `None` (the default),
  reduces all dimensions. Must be in the range
  `[-rank(input_tensor), rank(input_tensor))`.
  