QOpenGLContext::getProcAdress is not guaranteed to return nullptr on all platforms

Documentation for QOpenGLContext::getProcAdress (https://doc.qt.io/qt-6/qopenglcontext.html#getProcAddress) for both Qt 5 and Qt 6 states that this function is supposed to return nullptr in case the requested function is not found.

This  behavior, in fact, is platform and OpenGL implementation dependent which makes the statement misleading. This call in some cases boils down to glXGetProcAddress() call which does not return NULL / nullptr in case of a non-existing or unsupported function request.

In order to reproduce, request procedure address with an arbitrary string under Linux, you will not get a nullptr returned. 

Please remove the misleading statement in the documentation or mark as platform dependent.