QtSerialPort module is not part of Qt 5.0. It was added to the upcoming release Qt 5.1.
You are looking at the documentation snapshot for the upcoming release Qt 5.1. See http://doc-snapshot.qt-project.org/qt5-stable/qtdoc/index.html.
The documentation for Qt 5.0 is here: http://qt-project.org/doc/qt-5.0/qtdoc/index.html.
If you still want to try QtSerialPort, the RC1 of Qt 5.1 was just released. You can download it here: http://download.qt-project.org/development_releases/qt/5.1/5.1.0-rc1/