virtio-gpu api: Add a comment on VIRTIO_GPU_SHM_ID_HOST_VISIBLE

This provides a description of how the kernel driver uses the
shmid to determine capabilities.

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201119010809.528-3-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>