build-export: Document that NOW is a valid timestamp
build-export: Document that NOW is a valid timestamp

We may not want to guarantee all the formats that this
datetime parser accepts, but NOW seems useful enough and
is already mentioned in flatpak-build-commit-from(1).

Closes: #2498
Approved by: matthiasclasen

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