message: Make tf.where documentation clearer.
diff: @@ -3191,7 +3191,7 @@ def where(condition, x=None, y=None, name=None):
  Returns:
  A `Tensor` with the same type and shape as `x`, `y` if they are non-None.
  - A `Tensor` with shape `(num_true, dim_size(condition))`.
  + Otherwise, a `Tensor` with shape `(num_true, rank(condition))`.
  Raises:
  ValueError: When exactly one of `x` or `y` is non-None.
  