If they aren’t documented for the POST request payload, you can’t rely on being able to set them like that even if it happened to work.
I suspect the thing to do is to look at the topic_url in the response after creating category, then get a single topic using the id from that URL. Look at the post_stream in the response which has an array of posts, I imagine that will have one element.
Get the id of that post and then update a single post to add your description. That should trigger the category description being updated the same as editing the post in Discourse, same first line rule.