vfio: add documentation for posted write argument

Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250616101314.3189793-1-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

include/hw/vfio/vfio-device.h

@@ -240,6 +240,7 @@ struct VFIODeviceIOOps {
      * @off: offset within the region
      * @size: size in bytes to write
      * @data: buffer to write from
+     * @post: true if this is a posted write
      *
      * Returns number of bytes write on success or -errno.
      */