QDoc warnings for missing documentation for QOpenGLFunctions_* classes

These new public QOpenGLFunctions_?_? classes introduced for Qt 5.1:

http://doc-snapshot.qt-project.org/qt5-stable/qtdoc/classes.html#o

Do not have documentation for their member functions, and QDoc generates thousands of warnings about them.

Obviously these cannot and will not be documented in the scope of Qt, as they are simply wrapper classes for specific OpenGL version/profile APIs.

However, we need to suppress the warnings somehow - for example, introduce a new qdoc command used in relation with \class that generates a default documentation block for a node if such doesn't exist.