Document VirtIO-GPU fence-passing feature

[digetx]

Document new fence-passing feature of VirtIO-GPU. There are AMDGPU [1] and virtio-intel [2] native contexts that are making use of this feature. The Linux kernel driver RFC patch was posted on [3], it will wait until the spec update is accepted.

[1] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21658
[2] https://gitlab.freedesktop.org/digetx/mesa/-/commits/native-context-iris
[3] https://lore.kernel.org/dri-devel/20231007194747.788934-2-dmitry.osipenko@collabora.com/

Please merge the doc update: https://lists.oasis-open.org/archives/virtio-comment/202401/msg00145.html