You don't need anything fancy: the following contains a new line (two, actually):
<summary>Tootsie roll tiramisu macaroon wafer carrot cake.
Danish topping sugar plum tart bonbon caramels cake.
</summary>
The question is, why isn't this newline having the desired effect: and that's a question about what the recipient of the XML is actually doing with it. For example, if the recipient is translating it to HTML and the HTML is being displayed in the browser, then the newline will be converted to a space by the browser. You need to tell us something about the processing pipeline.