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.orgSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

include/uapi/linux/virtio_gpu.h

@@ -115,6 +115,10 @@ enum virtio_gpu_ctrl_type {
 
 enum virtio_gpu_shm_id {
 	VIRTIO_GPU_SHM_ID_UNDEFINED = 0,
+	/*
+	 * VIRTIO_GPU_CMD_RESOURCE_MAP_BLOB
+	 * VIRTIO_GPU_CMD_RESOURCE_UNMAP_BLOB
+	 */
 	VIRTIO_GPU_SHM_ID_HOST_VISIBLE = 1
 };