Doc: Fix QDoc warnings for QOpenGLFunctions and QOpenGLExtraFunctions

    - Use \a commands when listing the arguments for OpenGL methods
    - Document functions taking a single argument of type 'void'
      verbatim. Otherwise, QDoc refuses to generate their docs
    - Add constructor docs for QOpenGLExtraFunctions (copied from
      QOpenGLFunctions).

Change-Id: I64a436365572a80319ba0a8eaba2f5d9b751e84d
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>