doc: Mention that flatpak-run can be used with runtimes
doc: Mention that flatpak-run can be used with runtimes

The ability to use flatpak-run to enter into a shell in a runtime was
added in flatpak 0.6.13 but the docs weren't updated to reflect that.
This commit updates the flatpak-run documentation to explain the
functionality.

matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak