That description is only shown to new users and visitors. The idea is that you know the categories after using a forum for some time, so it is hidden for staff users and users with a higher trust level
I don’t think there is a setting. If there was one, I would expect it to be included in that code snippet.
I haven’t tried if it still works, but you could try
I actually do not miss the descriptions in the dropdown. If I want a list of all categories with their descriptions, I look at /categories (or the banners here at Meta). I like that the dropdown is short, so I do not have to scroll that much.