Elaborate on the expected behavior of QSG_RHI_DEBUG_LAYER

https://doc.qt.io/qt-6/qtquick-visualcanvas-scenegraph-renderer.html#rendering-via-the-qt-rendering-hardware-interface

The table in the section has an item called QSG_RHI_DEBUG_LAYER.

The description of it says "Where applicable (Vulkan, Direct3D), enables the graphics API implementation's debug or validation layers, if available, either on the graphics device or the instance object. For Metal on macOS, set the environment variable METAL_DEVICE_WRAPPER_TYPE=1 instead."

I actually tried to make it 1 on my Qt Creator and ran one of the sample apps from Qt Quick 3D. But it turned out nothing was output. 
Now, even if that's the expected behavior and something is supposed to be output only when anything is wrong, I think it should also be stated in the description.

In anyways, it'd be helpful to write what the user is supposed to see on the screen if it's enabled.