Product Update emails are under the system emails submenu of emails.
You can delete the variable {topic content} from the email and the clients will just receive the post title. Which is what I did, until today.
After some trial and error, I discovered the variable {topic content} can hold 590 characters. So for my next product update and going forward until we get some enhancement to this variable, I will begin my Product updates with a 590 character paragraph (as the variable can hold no formatting) describing the contents of the product update and reintroduce the variable to the system email.
Thought I would share the results of my trial and error discovery :)