doc/flatpak-config: Improve synopsis
doc/flatpak-config: Improve synopsis

Make it clear that the command can be used a few different ways, and the
option used determines the needed positional arguments.

Closes: #2361
Approved by: matthiasclasen

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