Add comment about how the deferred callback for AC_FOUND_DEVICE we

Add comment about how the deferred callback for AC_FOUND_DEVICE we
generate for a race where a device goes away, we start to tear down
the periph state for the device, and then the device suddently
reappears. The key that makes it work is removal of periph from the
drv list before calling the deferred callback.

Hat tip to: mav@