There used to be a bug in the edit feature which prevented descriptions being edited. However, the bug was fixed in Mastodon version 4.1.0, so as long as your server is running 4.1.0 or higher then you should be able to edit descriptions just like you would edit the rest of the post.
You can see which version your server is running by going to your server’s website and looking at the bottom of the screen. If you’re on a phone browser, you may need to tap the ⋯ icon and then scroll to the bottom of the screen.
If your server is still running an older version of Mastodon, there is a workaround which does let you add descriptions through editing:
- Edit the post
- Delete the image
- Re-upload the same image
- Add the text description
- Save changes to the post