qt5-doc 5.9.1-2 (amd64 binary) in ubuntu bionic

 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.