manpage: Refer to flatpak-metadata documentation
manpage: Refer to flatpak-metadata documentation

The documentation for extension info can be found there.

See #1341.

Closes: #1927
Approved by: mwleeds

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