message: Add memory warning for broadcast_to docs.
diff: @@ -38,5 +38,14 @@ tf.Tensor(
  In the above example, the input Tensor with the shape of `[1, 3]`
  is broadcasted to output Tensor with shape of `[3, 3]`.
  +
  +When doing broadcasted operations such as multiplying a tensor
  +by a scalar, broadcasting (usually) confers some time or space
  +benefit, as the broadcasted tensor is never materialized.
  +
  +However, `broadcast_to` does not carry with it any such benefits.
  +The newly-created tensor takes the full memory of the broadcasted
  +shape. (In a graph context, `broadcast_to` might be fused to
  +subsequent operation and then be optimized away, however.)
  END
  }
  