message: Update doc string for tf.count_nonzero to add string type
diff: @@ -1467,7 +1467,8 @@ def count_nonzero(input_tensor,
  ```
  Args:
  - input_tensor: The tensor to reduce. Should be of numeric type, or `bool`.
  + input_tensor: The tensor to reduce. Should be of numeric type, `string`,
  + or `bool`.
  axis: The dimensions to reduce. If `None` (the default),
  reduces all dimensions. Must be in the range
  `[-rank(input_tensor), rank(input_tensor))`.
  