HAM allows for this via the Maemo-Upgrade-Description field in the package's control file.

However, it requires the packager to update that as well as the debian/changelog file that you are expected to update (and doing so is required to change the version). I used to use it, but then I'd forget to update that field and updates would show the previous version's changelog (as it was rare for me to need to change debian/control again afterwards, but I'd be required to change debian/changelog as explained above).