Currently the support docs for this function read:
(string) (Required) The URL of the image to download.
(int) (Required) The post ID the media is to be associated with.
(string) (Optional) Description of the image.
Default value: null
(string) (Optional) Accepts ‘html’ (image tag html) or ‘src’ (URL), or ‘id’ (attachment ID).
Default value: ‘html’
Unfortunately when they say “Description” they really mean “title” of the attachment, and not the field named “description” that’s on every attachment page. There is no way that I can tell to add a description to your attachments when using media_sideload_image.