You're right in that this varies by site, so a network-wide description is of limited use. But I think this also will vary by time, due to each moderator's skill level, interests, and time available to moderate. So I don't think a permanent document posted to each site would be useful in the long run. 

I think a good solution is to have the current moderators speak to what they do at the start of an election. Have another meta post added to the list of things the CMs create when an election starts. The meta post will simply request each moderator post something that outlines what they do as a moderator.

This way, the nominees will have a useful reference to work from. It will provide them with an idea of what it's like to be a moderator, and how they would fit into the existing team. It'll especially help them to find gaps in the current moderation coverage, which they can aspire to fill.  And it'll provide a place for nominees to publicly discuss with the existing moderators on how moderation works.  Later on, it can provide a handy reference during future discussions on how moderators are moderating.