portal: Document the new sandbox options
portal: Document the new sandbox options

This adds missing docs for sandbox-expose-fd, sandbox-expose-fd-ro and
sandbox-flags.

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