[documentation] - enumerate `api/spells/{index}` -> `damage` field

[ecshreve]

Currently the damage field in the Spell schema that describes response from this endpoint looks like this:

damage: {
  damage_at_slot_level: {
    <any-key>: any
  }
  damage_type: {
    index: string
    name: string
    url: string
  }
}

but this isn't entirely accurate. this field may also include a field named damage_at_character_level. would be nice to clean this schema up so it's fully accurate. here's some links that might be helpful

https://swagger.io/docs/specification/data-models/data-types/
https://swagger.io/docs/specification/describing-responses/
https://swagger.io/docs/specification/data-models/oneof-anyof-allof-not/