Document guest kernel requirement

[nirs]

Reading libkrun for macOS docs, it is using venus, which seems to require kernel 5.16:

The Venus driver requires supports for

VIRTGPU_PARAM_3D_FEATURES
VIRTGPU_PARAM_CAPSET_QUERY_FIX
VIRTGPU_PARAM_RESOURCE_BLOB
VIRTGPU_PARAM_HOST_VISIBLE
VIRTGPU_PARAM_CROSS_DEVICE
VIRTGPU_PARAM_CONTEXT_INIT

from the virtio-gpu kernel driver, unless vtest is used. That usually means the guest kernel should be at > least 5.16

But krunkit or libkrun do not document anything. I think we should document guest kernel requirement for specific features like GPU acceleration.