message: [XLA] Document the iota_dimension attribute for kIota
diff: @@ -1467,9 +1467,10 @@ transfer. Creates a rank 1 array of values starting at zero and incrementing by
  one.
  Arguments | Type | Semantics
  ---------- | --------------- | ------------------------------------
  +---------------- | --------------- | ------------------------------------
  `type` | `PrimitiveType` | type U
  `size` | `int64` | The number of elements in the array.
  +`iota_dimension` | `int64` | The dimension to increment along.
  ## Map
  