Hello ran into this issue as well in a 3.9 environment. Seems to be introduced in 3.8 as a request of someone who found that it was inconsistent to other descriptions which show when viewing the activity. However it does not seem to be implemented with an option to display on course page or only on first page of the book. I will ask Helen Foster to reconsider.
Link to tracker issue: https://tracker.moodle.org/browse/MDL-62835