What the title says. The docs just say "you can add customized fields!" but nothing actually explains what format you're supposed to put things in so that, eg. you can have actual text show up in the multiple choice fields.
Sorry about this, it's part of the user interface I never fully finished so it's incredibly confusing.
In the meantime, the format can be set in "configure field settings" with a line for each choice containing comma separated values:
The image_url and description are optional. id is a unique string that you can use to preserve responses if you decide to re-order or re-name them. You can just use an incrementing number:
1,First choice,Description about first,
2,Second choice,Description about second,