Announcement list: specify plain txt ver / html

I am sending an html message to an announcement list, but also want complete control over the look of the plain text version. DH says this is possible, but have found zero on how to make it happen.

Help?

Here's my source, from DH Newsletter Archive

http://wiki.dreamhost.com/index.php/V7.02_February_2005

"2. Announcement List Improvements!

Now, whenever you send an announcement you get to preview it first (in rendered HTML if it's an HTML announcement, and in plain text if it's plain text). Plus, for HTML announcements you can now completely specify
the plain text version, whereas before we would just strip out all the HTML tags for you and send a possibly very strangely formatted version."