Doc: Link to external docs in QOpenGLFunctions_* classes

These are wrapper classes that do not document their functions.
This change adds links to docs on opengl.org / khronos.org where
suitable, and changes the \brief commands to mention the correct
OpenGL version/profile.

Change-Id: I48154d5bce26f6753ca4400962939847c78a527d
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>