Guix manual warning about GPU Linux-libre compatibility

Guix manual warning about GPU Linux-libre compatibility

[Bone Baboon]

I found the Hardware Considerations section of the Guix manual very
helpful.  I was prepared for the issue of WiFi device compatibility.
However I was surprised by GPU compatibility.

I think it would be helpful to have a warning about GPU compatibility.
This would help someone who is looking at buying a computer by bringing
GPU compatibility to their attention.

I found the WiFi device warning with specific hardware that is known to
work very helpful.  Maybe an equivalent could be done for GPUs with
compatible GPUs something like:

* desktop good enough for Xorg server and Wayland window manager
* laptop good enough for Xorg server and Wayland window manager
* desktop very good GPU
* laptop very good GPU

Or maybe it just refers to h-node.

I appreciate that this extra warning may discourage some people from
trying Guix.  I think it would still be good to warn people about
potential problems so that they are not surprised.