install: Update --help output
install: Update --help output

Put brackets around the "LOCATION/REMOTE" argument to the install
command, because the remote name is now optional. It's difficult to
summarize all the forms supported by the command in one line, but this
seems more accurate than it was before.

Closes: #2288
Approved by: matthiasclasen

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