docs: Document environment variable handling
docs: Document environment variable handling

Mention how flatpak run handles environment variables,
and include the blacklist of variables we always override.

Closes: #2141
Approved by: alexlarsson

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