In Calibre you can downloud metadeta from sites like Goodreads, Amazon, B&N..., Edit metadeta > Downloud metadeta and covers\Downloud metadeta individually.
The metadeta you can download (Authours, Comments, Languages, Published Date, Publisher, Rating, Series, Tags, Title) and covers.
To update the metadeta in the device use
Kobo Utilities plugin (Update metadata in device library), after you update the metadata with Kobo Utilities the new comments will show in synopsis.
You can also add more information from other columns to synopsis, by using the template editor in Kobo Utilities,
example.