Install libayatana-appindicator-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libayatana-appindicator-doc
Description:
Ayatana Application Indicators (documentation files, GTK-2+ version)
A library and indicator to take menus from applications and place them in the panel. . This package contains developer documentation (GTK-2+ version).
Homepage: https://github.com/AyatanaIndicators/libayatana-appindicator
Version: 0.5.3-3
Section: universe/doc