Install libpanel-applet-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libpanel-applet-doc
Description:
library for GNOME Panel applets - documentation files
This package provides the documentation files for the GNOME Panel applet library functions.
Homepage: https://wiki.gnome.org/Projects/GnomePanel
Version: 1:3.26.0-1ubuntu5
Section: universe/doc