update Debian packaging script comments for source upload

As per [0], source only uploads are preferred. I believe they are now required
for migration out of testing [1]:

  From now on, we will no longer allow binaries uploaded by maintainers to
  migrate to testing. This means that you will need to do source-only uploads if
  you want them to reach bullseye.

  [0]: https://wiki.debian.org/SourceOnlyUpload
  [1]: https://lists.debian.org/debian-devel-announce/2019/07/msg00002.html

Github: related to #85 "release beta"