Nobody has an answer to such a simple question? Not even the Apple staff?
Even a "sorry, you can't use any formatting inside the description block of a descriptiveAlertTemplate; it's all disabled on purpose" would be better than no response at all.
Why is TVML such a poorly documented format?
Unfortunately, Apple staff doesn't seem to care much about answering any questions that are posted on this forum. Just have a look at the list of unanswered questions + questions answered with "please file as a bug/enhancement"...