message: add some more conversion methods to string_ops.md
diff: @@ -30,5 +30,10 @@ string tensor.
  ## Conversion
  * @{tf.as_string}
  +* @{tf.string_to_number}
  +
  +* @{tf.decode_raw}
  +* @{tf.decode_csv}
  +
  * @{tf.encode_base64}
  * @{tf.decode_base64}
  