docs: Clarify that --command is only for run
docs: Clarify that --command is only for run

This may not otherwise be obvious.

Closes: #2592
Approved by: matthiasclasen

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