message: Add "Raises" block to substr.
diff: @@ -111,5 +111,13 @@ length = [3, 2, 1]
  output = [b'hir', b'ee', b'n']
  ```
  +
  +Raises:
  +
  + * `ValueError`: If the first argument cannot be converted to a
  + Tensor of `dtype string`.
  + * `InvalidArgumentError`: If indicies are out of range.
  + * `ValueError`: If `pos` and `len` are not the same shape.
  +
  END
  }
  