I think there is currently no Documentation topic about category moderators.
You can find some information in the announcements related to the feature.
After enabling category moderation in the site settings, you should see a new dropdown in the settings of each category.
To find the category settings, navigate to a category and click the admin wrench at the top.
Then go to the settings tab. Below the “moderation” heading, you should see the dropdown to select groups who should be able to help moderate the category.
If you haven’t yet created the groups, take a look at Creating groups in Discourse