dir: Improve comments about deleting mirror refs
dir: Improve comments about deleting mirror refs

Make it clear which refs we delete and why, per the discussion here:
https://github.com/endlessm/flatpak/pull/200#discussion_r350053918

matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak