transaction: Add useful details to the docs
transaction: Add useful details to the docs

The guaranteed sequence of signals is useful information
for any user this api.

Closes: #2371
Approved by: alexlarsson

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