Add help to Seen

This could be fairly simple, but I'd love a little nudge regarding the optional 'index' mentioned. I'm not really sure what the index is; a valid timestamp in a particular format? Perhaps an IRC code? What is it?

So far I have:
> Ask the bot when it last saw someone with "seen [nick]". Optionally you can ask for a log entry at a specific index with "seen [nick] [idx]".

It would be great to add an example of a valid index here.