I think the "indexing" refers to this:
if you look at your injectors in place on the engine, they are in fact not all straight up.....some of the injector plugs (at least on my S4) are very slightly 'off upright', so angled a tiny bit left or right..... which allows the connectors to fit back on as they came off on your particular car.
Some of the injector wires are fairly tight, there is not a lot of movement, especially as their original suppleness after 30+ years is way past its sell by date
so, as Tom says there is certainly no indexing mark, but they are not 'necessarily' all facing up dead straight....as he says "mostly up" is probably the correct term here.
it is just something to ease the refitting of the connectors to your particular car
that is what I think the manual is referring to, as indexing.
hey, I might be wrong, but I remember asking my injector service place to put them back on exactly as they came off the rail.