Install qt5-doc by entering the following commands in the terminal:
sudo apt update sudo apt install qt5-doc
Description:
Qt 5 API Documentation
Qt is a cross-platform C++ application framework. Qt's primary feature is its rich set of widgets that provide standard GUI functionality. . Qt Documentation contains the offline documentation for Qt 5. This package contains the complete Qt 5 API documentation (Qt's help format). . The documentation is provided in the Qt's help format and manpage format; the Qt's help format version can be viewed in conjunction with the Qt Assistant found in the qttools5-dev-tools package.
Homepage: http://qt-project.org/
Version: 5.9.5-0ubuntu1
Section: universe/doc